I am by no means an expert but here are a couple of comments:-
1. #You can get a silicone spray to spray the skids to make them a little more slippery but that is really addressing the symptom not he cause.
2. #There is a good article in one of the fishing mags this month ( either QFM or Bush and Beach, I think) that gives good advice on how to set up the trailer properly. #ie for an aluminium boat, the weight carrying should be done by the keel rollers not the skids. #The skids are just to steady the boat in place. #Best to read the magazine to get the drift.