While your running of course your momentum is going in a horizontal direction, the key is to transfer that horizontal mom. into vertical(up) mom. To do that, you should work on blocking, which is simply is the 'take off'. You should start blocking with your feet in front of you, so you get less distance but more height. Most people, just try to dive into their flip, but thats just gonna head you straight to the ground. Also, try to block with your hands above your head so your center of gravity gets risen, thus creating a higher axis to flip around. Try to block with your body as straight as it can be, from your toes to the tips of your fingers. This way the vertical momentum, has an easy path up, so to speak. Also, about tucking, all you should think about is tucking as MUCH as you can. Remember, the smaller you are the faster you flip. Well, I hope I helped some.
Key Points:
1. Block with feet in front of you
2. Block with your arms above your head
3. Keeep your body as straight as it can while blocking
4. Tuck as tight as you can
Hope I helped. :]