I went with 10' x 2 inch pvc, the gray electrical stuff, which works out very well. The boat slides easy enough on it and the pipe fits the channels in the bottom of the boat perfectly.
If you're looking for some awesome tie downs, I recently added these and love them!http://www.amazon.com/CargoBuckle-F1037 ... obuckle+g2
They're like your normal ratchet straps but retract into the housing when not used. You bolt the housing on the trailer, then never have to deal with reattaching/putting away your straps when the boat is off the trailer.