If your pedals are tough to remove, 1st spray the backside with some sort of liquid wrench type penetrating oil.After you do this give the axle a coulple of sharp raps with a metal hammer and punch(you want metal to metal contact to get some sharp vibrations/waves to move through the axel and threads where the axle has frozen itself to the crank.
Wait 20 minutes.Now get another set of hands if possible.Put a wrench on the pedal(as long a wrench as possible).Have one person hold the wrench and crankarm so when you turn the wrench, it won't want to turn the crank.This preson might want to wear gloves.You should get a heavy steel hammer, or a 2x4 and hit the wrench as far out as possible-give it a sharp rap(make sure the direction is correct).Make sure the wrench holder has the wrench pretensioned so it doesn't move and ding up the pedal flat when it is hit.
Be careful not to hit your assistants hands or hit the frame(don't ask why I'm adding this too obvious caution.).If you are working alone-be double careful(your hands).The pedal should move immediately, so make sure the hammer drop isn't going to ding anything when it suddenly loses resistance.
Luck,Charlie
PS I usually find it easier to use a heavier hammer or 2x4 moving more slowly than a light hammer moving faster.