Also make sure to change jobs every few years so you can move money from the 401k to an IRA without a tax hit. 401k's tend to suck - high fees, low returns, little competition. Most of the funds in your 401k are there because a fund salesman took an HR person out to a steak dinner.
When you change jobs, always move the 401k into an IRA and put the money into low cost ETFs.