Some low-hanging fruit:
Avoid artificially concentrated sugars. Don't drink soda or eat candy. Trans fats are almost certainly bad as well, (but are slowly being replaced in most foods). Don't drink too much alcohol. (Also, don't smoke)
The field of nutrition is a mess, but I think the above claims are genuinely noncontroversial, and can lead to larger improvements than any other general or exotic nutritional advice.
This is a thread where people can ask questions that they would ordinarily feel embarrassed for not knowing the answer to. The previous "stupid" questions thread is at almost 500 questions in about a month, so I think it's time for a new one.
Also, I have a new "stupid" question.