3M Adhesive Remover works very well. I bought a quart can with a cap with a squirt tip. After getting the decals off, I used an old towel. I held the towel below a section of the adhesive and slowly moved the tip of the can across and above the adhesive letting the liquid "sheet" down over that section of adhesive, soaking into the towel below. Then using that saturated towel, I could easily wipe off that section of adhesive. Basically just melts away. Keeping the towel moist with remover is essential to make it easier.
Have heard that mineral spirits works well also. That may be what the 3M is composed of. Probably cheaper.