Use a heat gun or hair dryer to heat up a small region of the decal. This will make the decal more pliable and reduce the adhesive bond.
Note: Be careful not to overheat the bodywork or the paint may blister.
Starting at a corner, lift the decal away from the bodywork.
Use both hands as you get to the wider sections of the decal.
Go SLOWLY. Too much tension will cause the decal to tear.
The heated area will cool off after a few minutes, causing the adhesive bond to increase again. You will notice that the tension will increase and the decal will tend to tear easily.
Simply repeat Step 1 and continue.
After the decal is removed there may be adhesive residue remaining. Use an adhesive remover to remove residue. We really like the citrus-based adhesive removers like GooGone because they are non-toxic, bio-degradable, and can be readily found at hardware, department, and grocery stores.
Note: Older decals tend to become brittle over time and keeping it warm is very important. Also, some decals simply don't comply with the rules and take longer to remove.