Things like freecreditreport.com are okay, but don't do much other than send alerts. I used them for a while but they still miss a lot. You are better getting with one of the insurance/protection groups. These guys do a lot more than monitor, they insure you against loss and clean up the public information about you that is most easily accessed. One major bonus is that this stops most of the unsolicited credit offers you get via mail or phone.
Here is a good example of a total ID theft prevention company, albeit there ar emany others:
http://www.zanderins.com/idtheft/idtheft.aspx