Need help with grinds, like boardslide, darkslide, 50-50, etc..?

everytime i try to ollie onto a rail, right when im at the rail, i always mess up. i usually pop my back foot, but i always lean back. i cant seem to be able to even out to do a 50-50. and with the darkslide, it always slips off of the rail. HELP! anyone that has the best answer and explains whats wrong with all of my problems will be chosen as best answer