90% of the time, your rear brake is just something for your right hand to do while your left hand/front brake does the work. It doesn't add any stopping power, and can cause the rear wheel to lose traction. As others have said, shift your weight [what feels like] way back (look at some pictures of DH mountain bikers and how far they get back... clearly you don't need to go to that extreme, but it illustrates the point nicely), and don't touch the rear brake. The exception is that when it is wet or icy and there is a risk of the front wheel skidding. You rarely can recover from a front wheel skid, but if just the rear is skidding you're usually fine, so in these conditions use the rear more.