I know it's weird to do when you're under 30, but boy am I glad I've been putting money in mine since 1995 (I was 24 at the time). I know a number of people who are now living on what they saved during their employed years, adn the earlier you start the better off you are in the long run.

And if you start young, you can be aggressive in the early goings and then adjust what you're putting money into as you go along, getting more conservative with your holdings as you get older.

(yeah, I know, it's still weird. Even when it makes perfect financial sense.)