you pretty much just have to wail on the things. My rears were especially snug when I did my brakes recently.
My technique is this, with a block of wood on the wheel, hit it with a sledge. But, instead of rolling the wheel, or moving the impact point around the wheel, i.e. 1 o'clock, 2 o'clock, 3 o'clock, etc, etc, try alternating your wacks on opposite sides of the tire, ex. 3 o'clock, then 9 o'clock, then back to 3 o'clock, then 9, you get the idea. Once it starts to come a little loose, move and alternate between 12 o'clock and 6 o'clock. This method hasn't failed me yet.
And, oh yeah, like the others have mentioned, anti-sieze those mo-fo's before you put them back on!!!
I jacked the rear end up, removed the lug nut cover, removed the 4 lug nuts, removed the center metal hub cover, and then attempted to remove the rear tire. I couldn't get the thing to budge! Anyone else have this problem? I had my tires rotated about 7,000 miles ago and clearly the tire shop removed the tires so why was it so difficult for me to remove them? Is there something I'm missing? Thanks for the help! 
also try putting the lugs back on with an impact wrench and tat might loosen up the stuck wheels.