Practice your braking response times and distances. At first, get comfortable making quick and controlled decelerations on straight, flat roads and in dry conditions. Then, graduate to taking on small descents on your bike, and feel how much pressure you need to make a full, controlled stop. Work your way up to slightly larger descents, and always retain control of your bike.
How To: Keep your weight on your rear wheel, relax your upper body and firmly squeeze—not grab—your brakes. You'll want to control your speed with primarily your rear wheel so you don't lock up your front wheel and flip over your bars.