Since I'm going through this process myself, I've been writing articles on eHow and Bukisa about how to properly allocate 401(k) funds. Because I like dealing with money so much, I'm having a lot of fun with all this 401(k) business, but I fear people who aren't very knowledgeable in the area may be confused by my writings. If you read any stuff I write and get confused, please comment!
Pressing on to how to properly allocate 401(k) funds. If you haven't read the above articles, please do. The advice below is supplimental to the advice in those articles.
This article, from Bankrate.com does a better job explaining asset allocation by age than I ever could. Basically, the younger you are, the more risk tolerance you have, so the riskier your investments. You have plenty of time for the markets to make up for any downturns. In the same way, the older you are, the less time you have for the markets to come back from any downturns, and the less risky you want your investments to be.
Did you check your 401(k) yet? Do you know where your money is? All of it? Do you have a good idea of what all the symbols mean? This is your responsibilty. Take advantage of the time you have and the fact that we've got sideways markets right now. Reallocate your assets.