WARNING: Involves glass, fire and solvents… If you can barely tie your shoelaces without a paper cut, you may end up killing yourself or cutting part of your jaw off by doing this. So please be careful.
UPDATE: New video alternative at the bottom is a little safer
The first DIY version below is good however lots of people have mixed results, a better idea (although one that costs more money) is to simply buy this > http://amzn.to/1JEEN6F
Step 1 – Grab a beer bottle preferably with thick glass such as corona bottles. Tie a string just above the label on the empty bottle.
Step 2 – Keep the string tied and soak it in lighter fluid.
Step 3 – Put the string back on the bottle and hold it horizontally. Light the sting rotating the bottle so the flame spreads. You should hear the bottle crack slightly in about 10 seconds.
Step 4 – After you hear the crack, pour cold water on the string and the top of the bottle will fall off.
Step 5 – Now grab sandpaper and sand the edges of the bottle till it is smooth.
via College Envy
Alternative & safer solution:
You will need one of these > http://amzn.to/1JEEN6F