Scraps of various size lumber. Cutting ends (or just finding scraps with angled ends) helps a lot. Also, just having some scraps of 3/4" plywood helps make nice small "stair steps". The 1.5" of dimensional lumber can be a bit much.
In most cases leveling side to side just takes a few minutes..... and I'm usually pretty motivated since once I'm done it's time to crack a beer. I've gotten to know each of my trailers and slides and how they respond when the slides are extended. I try to go about 1/2" high on the slide side and it comes out just about perfect.
