If it's a standard, put the car in gear and that will keep the crank from turning. Depending on how tight it is, you might have to put something in front of the wheels because you can move the car by turning the crank if it's tight enough. If it's automatic, you might have to pull the flex plate cover off and wedge a wrench or screwdriver on the flexplate to keep it from turning.
If you have an impact use that! It took the nut loose with no problem, but you have to remove the radiator to get the impact to fit. I also heard its best if you put your car in 5th gear if its a 5spd
I had to use the impact wrench man. Two people couldnt yank that bolt loose. Then sometimes even with the car in gear the damn car started to rock back and furth on the jack stands. (scared the piss outa me too). So in short I used the impact wrench.
Or you can just put two of your pulley bolts in and then put a crow-bar in between them.. thats what i did... Next time i do it im saving time with an impact.