The easiest way is to get rubbing alcohol (we have it in a spray bottle but no need to go to extremes), apply it to the glue and rub it off with a lint-free piece of cloth. Easy!
I found they come off a ton easier if you heat them up first. I took my sisters blowdryer and warmed them up, peeled them and then use your finger to push it all together or rubbing alcohol/nail polish remover