You need to use your shoulders more than your back. For guys this is hard. Their shoulders just don't tend to be as flexible as girls. Work on doing a bridge with your shoulders pushing as far over your hands as possible.
Backbend -- Stand up as straight as you can with your feet shoulder width apart. Look back and try to stretch your arms behind your head as you go down. Do it onto an elevated surface for a while until you can consistently do it using your shoulders instead of your knees.
Back walkover -- Again, start standing up as straight as you can. Reach for the ground with your shoulders -- not your back. As soon as you touch your hands to the floor, lift your leg over. Keep pushing your shoulders over your hands until you are in the handstand. Also, make sure the leg that you are kicking is straight. If you kick a bent leg, it will pull you back down to the floor. Good luck.