throwing in a Sheldon to better explain front vs. rear braking
rear brakes can be replaced by either fixed gear skidding or coasters
I find rear brakes on fixed gears very awkward and never use them, unless flip flopped to the freewheel
new to fixed so still going to the freewheel from time to time
the guy that mentioned modulation, arm bracing and rear center of gravity to prevent endos is right on
however abrupt braking at 20+ mph will likely lead to an endo due to simple physics

[h=2]Braking--Front, Rear or Both?[/h]Since your bike has two brakes, one for each hand, if you want to stop as safely as possible, you need to pay attention to how you use each of them.
[h=3]Conventional Wisdom[/h]Conventional wisdom says to use both brakes at the same time. This is probably good advice for beginners, who have not yet learned to use their brakes skillfully, but if you don't graduate past this stage, you will never be able to stop as short safely as a cyclist who has learned to use the front brake by itself.
[h=3]Maximum Deceleration--Emergency Stops[/h]The fastest that you can stop any bike of normal wheelbase is to apply the front brake so hard that the rear wheel is just about to lift off the ground. In this situation, the rear wheel cannot contribute to stopping power, since it has no traction.
[h=3]Won't I Go Over The Bars?[/h]The rear brake is O.K. for situations where traction is poor, or for when your front tire blows, but for stopping on dry pavement, the front brake alone provides the maximum stopping power, both in theory and in practice.If you take the time to learn to use the front brake correctly, you will be a safer cyclist.Many cyclists shy away from using the front brake, due to fear of flying over the handlebars. This does happen, but mainly to people who have not learned to modulate the front brake.The cyclist who relies on the rear brake for general stopping can get by until an emergency arises, and, in a panic, he or she grabs the unfamiliar front brake as well as the rear, for extra stopping power. This can cause the classic "over the bars" crash.Jobst Brandt has a quite plausible theory that the typical "over-the-bars" crash is caused, not so much by braking too hard, but by braking hard without using the rider's arms to brace against the deceleration: The bike stops, the rider keeps going until the rider's thighs bump into the handlebars, and the bike, which is no longer supporting the weight of the rider, flips.This cannot happen when you are using only the rear brake, because as soon as the rear wheel starts to lift, the rear wheel skids, limiting its braking force. Unfortunately, though, it takes twice as long to stop with the rear brake alone as with the front brake alone, so reliance on the rear brake is unsafe for cyclists who ever go fast. It is important to use your arms to brace yourself securely during hard braking, to prevent this. Indeed, good technique involves moving back on your saddle as far as you can comfortably go, to keep the center of gravity as far back as possible. This applies whether you are using the front, rear or both brakes. Using both brakes together can cause "fishtailing." If the rear wheel skids while braking force is also being applied to the front, the rear of the bike will tend to swing past the front, since the front is applying a greater decelerating force than the rear. Once the rear tire starts to skid, it can move sideways as easily as forward.If you don't believe me, perhaps John Forester can convince you...see his Entry in the rec.bicycles FAQ on Front Brake Usage (Subject: 9.17). (Unfortunately, the maintainer of that site has a habit of breaking links, so you may need to go to the rec.bicycles FAQ index to find the article.) Or read John Allen's advice.Skidding the rear wheel also wears the rear tire very quickly. A single rear-brake-only stop from 50 km/h (30 mph) with a locked rear wheel can wear the tread of a road tire right down to the fabric!
[h=3]Learning to Use The Front Brake[/h]Maximum braking occurs when the front brake is applied so hard that the rear wheel is just about to lift off. At that point, the slightest amount of rear brake will cause the rear wheel to skid.If you ride a conventional bike, the best way to master the use of your front brake is to practice in a parking lot or other safe space, applying both brakes at once, but putting most of the effort into the front brake. Keep pedaling as you brake, so that your legs will tell you immediately when the rear wheel starts to skid. Squeeze, don't grab, the brake levers, so you can sense when this happens. Practice harder and harder stops, so that you will learn the feel of stopping fast, on the edge of rear-wheel liftoff.Test the brakes in this way whenever you are about to ride an unfamiliar bike. Some brakes are more sensitive than others, and you need to know the "feel" of the brakes.Once you are comfortable with the front brake, also practice releasing the brakes to recover control, until this is an automatic, reflex action. At a very low speed, apply the brakes hard enough that the rear wheel skids, or just begins to lift. When it does, immediately release the brakes. Wear your helmet.Some cyclists like to ride a fixed-gear bicycle, that is, a bicycle that does not permit coasting. When you brake hard with the front brake on a fixed gear, the drivetrain gives you excellent feedback about the traction at the rear wheel. (This is one of the reasons that fixed gears are favored for winter riding.)If you ride a fixed gear with only a front brake, your legs will tell you exactly when you are at the maximum brake capacity of the front brake. Once your fixed gear has taught you this, you will be able to stop any bicycle better, using the front brake alone.If you find the fixed-gear concept intriguing, I have a major article on Fixed Gears for Road Use on this site, and also a page of Fixed-Gear Testimonials from happy converts.
[h=3]When to Use The Rear Brake[/h]Skilled cyclists use the front brake alone probably 95% of the time, but there are instances when the rear brake is preferred:

• Slippery surfaces. On good, dry pavement, unless leaning in a turn, it is impossible to skid the front wheel by braking. On slippery surfaces, however, it is possible. A front wheel skid almost always leads to a fall, so if there is a high risk of skidding, you're better off controlling your speed with the rear brake.
• Bumpy surfaces. On rough surfaces, your wheels may actually bounce up into the air. If there is a chance of this, don't use the front brake. If you ride into a bump while applying the front brake, the bicycle will have a harder time mounting the bump. If you apply the front brake while the wheel is airborne, it will stop, and coming down on a stopped front wheel is a Very Bad Thing.
• Front flat. If you have tire blowout or a sudden flat on the front wheel, you should use the rear brake alone to bring yourself to a safe stop. Braking a wheel that has a deflated tire can cause the tire to come off the rim, and is likely to cause a crash.
• Broken cable...or other failure of the front brake.