http://www.tintdude.com has some tips and info

I would let a pro remove the tint that has defroster lines underneath it. Less chances of defroster line damage. On my non-Honda-Acura, I was going to remove my 10+ year old tint on my back window myself. I bought a portable steamer, as mentioned on the TintDude site. The guy who was going to tint it suggested that he removes the tint, since he has a technique that works. He said that if I do it, that he is not responsible for any small bubbles from debris, glue, etc that was from my removal job. He said that if he did the job, there would be less chances of that. He said that he would do the removal job for $70. For that low of price, I let him. The day of the removal and retint of the backwindow, I dropped the car off before he opened at 8:30am. He was done totally at 3:30pm. Removal, with retint of the backwindow, and a strip of tint on the windshield. He said it was one of the worst removal jobs so far.

Any other windows without defroster lines you can remove the tint yourself with some time and labor.