This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
There are several ways of stopping. The first one is to use
the brakes. This does not always work. Breaks can ice up,
a bit of water gets between the cable and its sheathing when
the warm afternoon sun shines on the bike. It freezes solid
after. Or the salt causes brake cables to break, etc. I
have had brakes work on one corner, but stop working by the
time I get to the next. I have several other means of
The casual method. For a stop when you have plenty of time.
Rest the ball of your foot on top of the front derailleur,
and *gradually* work your heel between the tire and the
frame. By varying the pressure, you can control your speed.
Be sure that you don't let your foot get wedged in there!
Faster method. Get your pedals in the 6-12 O'clock
position. Stand up. The 6 O'clock foot remains on the
pedal, while you place the other foot on the ground in front
of the pedal. By varying your balance, you can apply more
or less pressure to your foot. The pedal, wedged against
the back of your calf, forces your foot down more, providing
Really fast! Start with the fast method, but then dismount
while sliding the bicycle in front of you. You will end up
sliding on your two feet, holding onto the bike in front for
balance. If it gets *really* critical, throw the bike ahead
of you, and sit down and roll. Do not do this on dry
pavement, your feet need to be able to slide.
In some conditions, running into a snow bank on the side will stop you
quickly, easily, and safely. If you're going too fast, you might want
to dive off of the bicycle over the side. Only do this when the snow
bank is soft. Make sure that there isn't a car hidden under that soft
snow. Don't jump into fire hydrants either.