what i do is i lower them to my abdomen. that's far enough down my body that i can get a momentum rock up and then it's easy to put them down.
if you have the resources, another way is to grab two more benches or two chairs and put them on either side and put the dumbbells on that (bring them to your chest then roll to one side and rest the upper dumbbell, roll to the other and rest the other.
failing those, you might want to consider switching to the cable station.
oh - forgot the rock-n-roll: as you lower the dumbbells, ... it's kind of hard to describe - but as they're coming down, you're going up into a sit and guiding the dumbbells to the top of your thighs.
i'll look for a video to explain it better - here - check the very end of the clip (although the clip itself is impressive)
http://www.youtube.com/watch?v=8ktKWd8FxDg