I just took the chevrolet one off last weekend... I took a thin credit card and started it at the corner and went up along the thin side so I could get the whole sticker peeled in one long pull.
Basically after I got the side started, I just folded the non-sticky side of the sticker against the unpeeled non-sticky side and slid it across it (The way that I found that keeps the most goo on the sticker), and about every inch, fold the sticker over top of itself (if you don't, the sticker stretches and eventually rips). If it ever splits, make sure you take the credit card and even it back out w/ the rest of the sticker so it pulls evenly again (mine had some holes and scratches in it, so it separated a few spots). When you get the whole sticker off, I took wd40 on a rag and rubbed the sticky stuff off.
About the dealer thing... depends if it is a badge or a sticker. If it is a badge (mine was), I just ripped it off and cleaned the sticky stuff off w/ wd40. If it is a sticker, just do like the Chevrolet sticker.
Also, after you clean off the sticky stuff w/ wd40, you should probably wash the car and wax it (I don't think wd40 is good to leave on the paint for a long time). Taking the sticker off does make it a lot cleaner looking... Here is a pic of the back of my truck.
DogP