Ways you can do it:
Sticker Adhesive remover (Recommended)
Step 1: Heat (Cost Effective)
Step 1 Heat up the sticker for about 10 seconds (if it's a long sticker, do it in sections). The sticker should be soft now, and you can peel it right off! Some areas might be tougher, but that's what your fingernails for.
Step 2 You might have some sticker Adhesive left over on the window or car body. You can try heating this up, and wipe it off with a clean rag. You can also try some cleaner and wet it, and see if that removes the leftover sticker Adhesive.
This is a cheap way, using around the house tools, but it takes a lot longer.
Step 2: Sticker Adhesive Remover (Fast 'N Easy)
Step 1 Spray the Sticker Adhesive Remover on the sticker, trying not to let it run to much. Wait a few seconds, and peel of the sticker.
Step 2 If there's some Adhesive left after removing the stickers (happens on windows), spray the Sticker Adhesive Remover and wipe it off with a clean rag while it's still wet. It should come right off, as easy as cleaning off dirt.
Warning: This stuff ruins plastic, but it fixes itself in a few months. I got some on the plastic mirror holder on the driver-door rear-view mirror and it left lots of white spots, but they are gone now.
Step 3: Trim Removal
Step 1 Remove the trim. I used a screw driver, but you risk scratching the paint (I did once, fixed it up with some Dulicolor Quick Fix It Black Paint, my truck has so many scratches it doesn't matter anyway) but I don't recommend this. I hear some Goo Gone and fishing line can do the trick. If not, spray the 3M Sticker Adhesive Remover so it seeps under the trim.
Step 2 If you still have tape left on your car, spray the 3M Sticker Adhesive Remover and wait 20 seconds for it to soften up the tape. Then use your thumb and rub the tape off the car. It's fairly easy, but time consuming and you can hurt your thumbs.
Step 3 Be sure to clean the car good after that!
I don't recommend using a hair dryer on Step 2 because I burned my thumb trying it one day.