I always use a heat gun, or a hair dryer, if you don't have a heat gun. The hair dryer won't get as hot, but will work very well. Don't get the decal too hot, or it will tear easily. You will get the hang of it after a few trys. If you can find the right temperature, the glue will come off with the decal. If not, use any naturally based citrus cleaner for glue removal. I use 3M citrus cleaner in an aerosol can. It won't hurt the plastic, paint or anything else you get it on. I wouldn't use WD-40 if you are putting on new decals, as it will leave a film on the surface. The last thing you need is for your new decals to come off, or the edges to release.