FB is of course correct that there is nothing wrong with your rear brakes (at least not just because you can make the tire skid) but there is more involved than just modulating the front brake. The key is to use the rear wheel as an indicator, and to release pressure on the front brake when the rear wheel skids. But shifting one's body to the rear can greatly increase the braking rate without the rear wheel skidding. I'll not go into the full explanation of an "Instant Stop" here, as it's already done well at this link:
http://floridabicycle.org/chapter/firstcoast/?p=516

Please note that the above article and just about every other one that is similar owe much to John Forester, the originator of Effective Cycling. There are other very useful techniques and traffic strategies that commuters especially may find useful. I had the great fortune to take his Effective Cycling Instructor Course back in the late 70's. I also credit him with teaching me a more "effective" way to change a tire, even though I had been a mechanic for several years.

Sheldon also has some interesting views about brake usage at http://sheldonbrown.com/brakturn.html