These are cliche but very true.
Diversify. That way if one stock goes under, you'll take a hit but not being ruined.
Also, you generally want to think about growth versus risk(If someone tells you you can have high growth and no or minimal risk at the same time, they are lying. See Bernie Madoff.) When you are younger and starting to save, you want to tend towards higher growth investments that are somewhat riskier, because even if your portfolio takes some hits or goes down, you have many years for it to grow, and long term, sound investing is much more profitable than say bonds or storing money in the bank. As you get closer to retirement, you begin to move you assets into low risk lower growth options, like government bonds or conservative mutual funds, the logic being that since you want to use the money soon, you don't want to take a short term hit for long term gain.
Personally, I have mutual funds, money market funds, and Exxon-Mobile stock, though that was gifted to me by my grandfather. I may have to get out of Exxon-Mobile at some point in the near future if they don't adapt to inevitable eventual development of other energy sources.