well, here is exactly what i do when i roll, and after a lot of drops and practice i have my rolls down pretty well. I'm not saying their perfect (though you cant really ever perfect a vault) but there not bad.
FYI: i roll over my right shoulder.
when landing, i have my legs pretty straight, slightly bent, but pretty straight. i look at the ground the whole time i am in the air and when vaulting before the drop. then, when my feet touch the ground, i bend them and lean forward and i also some what support my weight with my left hand. As i am leaning forward and my legs are bent to about 40 degree angle, i push with my legs (
almost jumping diagonally) forward and tuck my head in and shoot my right hand under me towards my left hip (same angle i am going to be rolling.) The same time i am pushing with my legs and my left hand is still on the ground (to help brace myself) and my head is tucked in, i tuck my right shoulder in. Now, when i do the rolling part, i use my left hand when my right shoulder hits the ground to help take some of the impact off my right shoulder. Then, i lift my left hand and tuck it in against my chest while i am rolling. The push from your legs and the landing from the drop gives me enough momentum for the rest of the roll. When i am rolling i am almost in a cannon ball (like when you jump into a pool and make a big splash) position. Then when my right foot touches the ground i have enough momentum to push up with my feet and keep running. (sometimes i have too much momentum and i almost jump when i come out of my roll lol)
I used to have the same troubles with rolls, ninja. To prevent my head from hitting, i really tuck it in and my left hand that helps brace myself also helps my head from hitting the ground.
to roll straighter, try to really shoot your right arm under and across you so it is by your left hip, so you are pointing in the exact opposite direction than the direction you are rolling.
I don't do this exactly everytime, but, in my mind, this is how to do a perfect roll. and from watching David belles roll analysis (dang it, Asa beat me to it

) it seems like that is what he is doing. i hope this helps.
