I'd add a few things that I've learned over the years:
1) Always be invested in the market. Corollary, don't time the market. This is by far the largest mistake people make.
Investors typically pull money out at the bottom after they've suffered a physiologically devastating loss, like at the end of 2008 and hence they miss the rebound, like 2009-now. This isn't quite the same but it shows what missing the top 25 days in the market over the past 45 years does to your returns.
http://www.marketwatch.com/story/how-missing-out-on-25-days-...
If you are an investor you need to be in the market, period.
2) Accept that you will lose money some years. If you are buying index funds then you will get market performance, ex fees. Markets go down sometimes. Stay the course.
3) Don't look every day or you will go nuts.
Keep in mind that the largest draw down (top to bottom) will be larger than what the returns look like if you just look year over year. Ie if you look and see the S&P lost 28% in 2008, understand that if you watched the S&P every day of 2008 then it probably lost more than 28% from its top to its bottom but rebounded slightly at the end of the year to make the year over year loss less than the maximum loss.
4) Have some exposure to outside of the US markets. Consider the scenario of investing all your money in the company you work for. In a rough time for your company you get the double whammy of losing money and possibly your job at the same time.
Similarly to how you are told to not invest all your money in the company you shouldn't invest solely in the country you live in, same principle.
EDIT see child comment, I mangled the English language in point 4