Boardslides/lipslides on rails. Every time I commit I end up slipping out backward even if I am slightly squatting down to keep my balance.
Doesnt help to squat down if your shoulders are behind. Try to have your shoulders more forwards. If your head/shoulders are behind your knees, its really difficult to recover from too much backwards leaning.
Really, like in most sports where one might use the body for balance, like skateboarding, rollerbladers, BMX, motocross etc, theres a basic position we call "Attack/Neutral position".
Slight bend in the knees, and shoulders forwards so that you are prepared for anything.