Originally Posted by halfspeed
There are flats on the pedal spindles right next to the crank. Forget "righty tighty, lefty loosy" on pedals, the left one is reverse threaded. It's easy to remember which way to turn the wrench this way:
1) Move the crank so that it is parallel to the ground and pointing toward the front wheel.
2) Put the wrench on the flats so that it is pointing back to the rear of the bike and slightly angled up from the crank.
3) Pull the wrench down towards the crank arm using the arm for leverage.
This works on both sides. If they are just too tight, use a pedal wrench or a long open ended wrench of the correct size. (Usually 15mm.)
If they still won't budge, try some penetrating oil over night.
If you still can't move them, buy a new bike.
A 5 pound hammer on the wrench handle has never failed me. Has saved buying lots of new bikes. I never put em on too thight or without grease....honest! But there must be lot of gorillas with cheater bars installing them somewhere.