Brakes.

Use them both, but know that the front brake is far more powerful than the rear. Keep your weight back towards the rear of the saddle if you do hard braking with the front so you don't get pitched forward, but rear braking alone won't stop you nearly as quickly as using both brakes and excessive rear braking can cause your rear end to lock up and slide out in certain situations so get used to bringing both brakes into play.

There's a pretty good discussion in the braking technique section on this page: http://cycling.wikia.com/wiki/Brakes

-Dave

Andy, I'll second Dave's remarks and add that I believe every rider should practice hard braking in a safe, controlled environment before they need to slow or stop hard out on the road. Grabbing a big handful of both brakes can be pretty scary the first few times, definately something you should be familar with so you'll be ready when the need arises.

I've actually had riders tell me they don't use the front brake for fear of "going over the handlebar". This means they are riding with very limited ability to slow the bike: in a situation where hard braking is required, they will be out of luck. For most of us, of course these situations are few and far between, which is why I think some practice is essential.

I'm right handed and tend to grab the right brake out of habit. All of my bikes are set up for right handle to the front and left handle to the back, except for my daily rider which is reversed for some odd reason. So, on my daily rider, I use mostly the rear brake, and on all of my other bikes, I primarily use the front brake.

I do agree with dhk2 about practicing hard braking before you need it. Most bikes that I have ridden react differently to hard braking. I have had them from tracking straight all the way to the rear coming around during hard braking. It is a good idea to find out what yours is going to do sooner rather than later in an emergency stop situation.

Everyone is different. Certainly a lot of braking power can be had from the front. But I find the rear is more controlable and makes less of an impact on cornering. It's best to get braking done before corner entry and not good practise to brake in cornering, but sometimes you have to (on twisty descents) and I prefer to trail the rear through the turn than the front (front braking mid corner causes me to run wide). Of course in hard braking you'd need to use both and put as much of your weight to the rear as possible to avoid lifiting the rear wheel off the deck.

In Australia the standard is to have the left lever attached to the rear brake (singnaling turns across lanes of traffic, which is a right hand turn in Aus still gives you access to the more controlable rear brake). However, when I started racing I found myself instinctively grabbing at the right lever (front brake) in hairy moments. While I gradually learned to brake less, this also lead me to change to Euro/American style with right hand rear braking to give me a bit more control in those situations. It is now so ingrained that I will never go back (though I cop a bit of stick from others for being different).

I rode motorcycles for years so the right hand/front brake is set up on all my bicycles.