2 main things
protect your head
push with the back leg
I personaly still have not perfected my ukemi since the teaching staff in my dojo always point out that my arm collapses when it shouldn't I wish I can tape my self and see where I fault.
I have serious issues with ukemi, especially back rolls, but forward are easier. Put one hand so it points behind you, on the opposite side shoulder of the one you're rolling over. Then put the other one as support, so the fingertips meet. I cant do it one handed yet.