Keep your own timesheet
The U.S. Department of Labor has released an iPhone app called DOL-Timesheet. Here is the description from iTunes Preview: "This is a timesheet to record the hours that you work and calculate the amount you may be owed by your employer. It also includes overtime pay calculations at a rate of one and one-half times (1.5) the regular rate of pay for all hours you work over 40 in a workweek." By the way, the app is free, even though iTunes has a link to "buy and download."
If you have an Android phone, there are several free apps available to do the recording, if not the overtime pay calculation. I have not tried any of these, so I cannot warrant any of them. Go to the Android Market.