Here's my advice: always check Snopes before forwarding anything.

Just wanted to point out an implicit and not necessarily correct assumption, leading to poor-quality advice:

Suppose you know a not-very-smart person (around or below average intelligence)

It seems that you assume that intelligence is one-dimensional. In my experience, while there is a correlation, most people are smarter in some areas than in others. For example, a mathematical genius may be incapable of introspection and have to interest in rational thinking outside math. Let's take your example:

S/he read about rationality, has utilitarian inclinations, and wants to make the world better. However, s/he isn't smart enough to discover new knowledge in most fields, or contribute very much to a conversation of more knowledgeable experts on a given topic. Let's assume s/he has no exceptional talents in any area.

First, an "average person" does not read about rationality and has no "utilitarian inclinations". They often do want to make the world better if socially conditioned to do so by their church or by the TV commercials showing a sick child in the 3rd world whom you can save for a dollar a day or something. So, the person you describe is not "average".

Second, this "average person" might be (and likely is) intelligent in a way that does not show up on the IQ tests: he or she might be unusually good at running a corner store, or being a great parent, or whatever. Some of the talents may be latent, because they had no chance of being manifested. I would still call it "intelligence" by Eliezer's definition: ability to optimize the universe, or at least some small slice of it.

As a consequence, your advice is suspiciously indistinguishable from the one you'd give an "LW-smart" person. My inclination would be to find this person's area of aptitude and offer custom advice that plays to their strengths.

More actionable rules might be better such as:

Wear a seat belt when driving. Save 10% of your income through your pension plan's index fund option. Don't smoke. Practice safe sex. Sign up for cryonics.

It seems to me that the advice about Givewell has a lot of evidence behind it, but the rest of the advice doesn't have much evidence that it gives any benefit at all, for people of average intelligence or otherwise. It would be good to have a Givewell-like project that evaluated the costs and benefits of following various rationality advice.

I have a fairly wide variety of friends. Here's some advice I find myself giving often, because it seems to cover a lot of what I think are the most common problems. The wording here isn't how I'd say it to them.

Health and lifestyle

• Don't engage the services of any non-evidence based medical practitioner.
• If you have a health problem, you're receiving treatment/advice, and not obviously improving, get a second opinion. And probably also a third. (I am not in the US)
• Don't smoke cigarettes. If you already smoke cigarettes, buy an e-cigarette (like, right now), even if you're sure you'll never use it. Once you've bought it, do try to use it, even if you don't think you'll like it.
• Always use barriers for vaginal and anal penetration outside a monogamous relationship
• Use either barriers or hormonal birth control for p-in-v intercourse
• If you or your partner is having non-monogamous sex, get STI tests twice a year
• If you frequently feel depressed or anxious, see a psychologist who practices cognitive behavioural therapy. Do not see one who practices psychoanalysis (see "evidence based health care"). Expect to trial several psychologists before finding one who is a good fit for you (see: "if health care not working, seek second opinion").

Personal finance

• Don't buy cars new. There's an established value-point for depreciation vs maintainence costs for cars. Use that.
• Don't get a credit card
• Don't take personal loans unless the interest is dominated by the cost of having no liquidity (e.g. eviction)
• If you can't maintain a monthly budget, give yourself a weekly budget. If you still can't maintain a weekly budget, give yourself a daily budget. A budget over a shorter time period is much less convenient --- but failing to maintain a budget at all and going broke is less convenient still.
• Never gamble, including playing the lottery
• Don't invest in individual stocks, or pay someone else to invest your money in individual stocks.
• Comply with your local tax laws, including filing your taxes on time. Know that people smarter than you are being employed to catch tax cheats who are also smarter than you. You will likely lose.

Career

• Avoid "winner takes all" professions, e.g. sports, music, academia in fields with no industrial application, etc
• Judge potential careers by expected value, not best or worst possible outcome. Look at median salaries and working conditions.
• Only enrol in tertiary/graduate education with a specific career outcome in mind
• Recognise the true cost of tertiary degrees, in opportunity cost as well as course fees. e.g. a full-fee-paid PhD program with a modest stipend is still enormously expensive
• Recognise that reputational capital distorts labor markets. Teachers are paid low because they're well respected, not despite being well respected. The how-do-you-do value of having a well respected career is not that enduring, so it's probably over-valued. It's better to avoid careers with excess reputational capital.
• Consider careers in the allied health professions . These professions have the best job security: the aging population ensures rising demand, they must be performed locally, and most seem like unlikely targets for automation or disruption. They also offer lots of human contact, which many find produces good job satisfaction. But, because they don't require medical degrees, they are fairly neutral in reputational capital, so your peers don't just work themselves to death to win zero-sum positional games against you.

Law and the justice system

• Avoid dominance contests. Learn to display submission readily, meekly and convincingly.
• Regard police officers as people who have power over you. They do. Their power is broad, and not constrained by some set of written rules. If you challenge them to a dominance contest, you will likely lose. They will fuck you up. And even if you "win", it'll be a pyrrhic victory --- you'll still have been better off not getting into it in the first place.
• Power is not moral authority. Someone may have real power over you, with no legitimate right to it.
• Moral authority is not power. Even if you have the legitimate right to do something, someone may have the power to stop you.

"Study rationality anyway. Work harder to make up for your lack of intelligence." I don't think most of LessWrong's material is out of reach of an average-intelligence person.

"Think about exactly what people mean by words when they use them; there are all kinds of tricks to watch out for involving subtle variations of a word's meaning." Read Yvain's "The Worst Argument in the World"..

"Don't fall for the sunk cost fallacy, which is what you're doing when you say 'This movie I'm watching sucks [absolutely, not just relative to what you'd expect for the cost], but I'm gonna keep watching, because I already payed to get in.'"

"Your brain loves to lie to you about the reason you want to do something. For example, maybe you're thinking about moving to a new job. You don't get along well with one of your current coworkers, but you don't think this is a good reason to get a new job, so you refuse to take that into consideration. Your brain makes a bigger deal of minor advantages of the new job to compensate. Learn to recognize these lies."

Play to your strengths; do what you're best at. You don't have to be best in the world at it for it to be valuable.

Good things about this advice are (a) it has a fairly-sound theory behind it (Comparative advantage), and (b) it applies whether or not you're smart, normal or dumb, so you don't get in to socially-destructive comparisons of intelligence.

Read Yvain's Epistemic Learned Helplessness. You can be convinced of anything by good arguing, but forewarned is forearmed.

When in doubt, ask. The stackexchange network is great to get answers to questions.

skeptics stackexchange is for example great to get answers to general questions.
If you encouter a significant claim on the internet it's often a useful website to check. Recently I came about the claim that batteries follow something like Moore's law. I headed over to skeptics stackexchange and post a question.

Another useful habit is Anki. Especially if you don't trust your brain to remember information on it's own, let Anki help you.

look for tested methods that have significant positive results relevant to the area of interest

This part of advice needs to be more specific. For example which "positive results" should be trusted and which not. Because everyone who wants to sell you something will tell you about "positive results".

I'm not sure how much raw intelligence matters. If a person who's average intelligence stays with a problem which doesn't get much attention for 10 years I see no reason why they shouldn't be able to contribute something to it.

Being intellectual means staying with intellectual problems over years instead of letting them drop because a new television series is more important.

I find your third point for practical advice to be significantly dis-charitable to someone of average intelligence. There are people that miss obvious patterns like, "This person gives bad advice," but I think people of average intellect are already well equipped to notice simple patterns like that.

I don't believe this is a coherent set of general advice that can be given here. What specific details and methods of rationality any given "average" person is missing, and what specific cognitive biases they suffer from most severely will vary too widely to get good coverage with a few short points. My approach would be to work on an individual basis to determine what's causing the most problems for each person and address them accordingly. This may seem highly inefficient, but remember that success stories are told and retold virally as each new person has experiences that confirm the wisdom:

That sounds a lot like what I went through. What really helped me was...

There are far too many average people for me to expect a single centralized fault will be considerably effective.

One of the most important steps to becoming more rational for an average person would be to disentangle themselves from the default goals / values imposed by society or their peers. This would free up a lot of time for figuring out their own goals and developing relevant skills.

An average person could go far with instrumental rationality techniques like those taught at CFAR. Exercises like goal factoring and habit training don't require a high capacity for abstraction, only willingness to be explicit about one's motivations. For accumulating factual knowledge, spaced repetition software could be very useful.

My 3 pieces of advice, for someone already convinced that something fairly accurate is going on, would be:

1) Try to sign up for cryonics before dying.

2) When donating to charity, use the recommendations of an organization like GiveWell ("the practical approach"), or donate to a charity working on existential risk ("the 'taking ideas seriously' approach).

3) (My one best piece of rationality advice) Other people have good reasons for their actions, according to themselves. That doesn't mean you'll think they're good once you find them out - but it does mean you should try to find them out.