The best thing that worked for me was to just keep them wet before sliding in. Have a passenger in the boat splash on the rails, or lean over your watercraft before sliding in.
Also try to do it in one smooth motion. I stretched the back muscles plenty of times trying to pull my waverunner in after stopping mid-way.
Good luck and Enjoy it, these things are great. I just put mine away for the winter. It will be on the for sale block come spring.