Pumpkin Roll Technique
I've seen 2 tricks to help.  One is to pre-roll the roll while it is still hot, just out of the oven.  You will sometimes get cracks when unrolling the roll, but they are less visable as the cracks originate on the inside of the roll.  Once you roll it back up, they tend to disappear.  

The other is to just bake it on a piece of parchemnt paper and use that to lift, roll and support the pumpkin cake.  As you lift the parchment paper, you roll the cake down into itself and it almost rolls itself as you lift the paper.  Just have to keep it moist, but not too moist.  

Good Luck!