A heat gun and 1" wide straight edged dull putty knife works for me. Takes about 5 minutes/tire for a slick. Work slowly and let the heat do the work for you. Don't hold the gun too close and keep moving it around.
I've also had good luck with a small 1000W hairdryer that got used in a pinch when I needed to rebalance a tire that lost a wheel weight, and I left the heat gun at home. This takes a little more time and it helps if the tire is still warm i.e. right off the track, but in a pinch, leave the tire out in the sun for an hour to warm up.