Use the heat to soften the glue that the sticker uses to adhere. I use a heat gun set on low and it works great.
So, heat a section of the sticker for a few seconds, start peeling the sticker from a corner in the heated area. As the sticker lifts, continue heating the next section of the sticker and continue pulling the sticker up off the plastic with your other hand. You'll know when you have the heating correct when the sticker lifts smoothly with out stretching or breaking into pieces. The goal is to get it off in as big of pieces that you can.
after the decal is off, and the glue residue remains, use
goo gone to get the glue removed.
do not use goof off