If you wait too long to pop, you'll know it. You'll be able to tell by the graceful way you'll be tumbling down the steps, or off the curb. If you do this, don't worry about it - wipe off the blood, re-attach your face and try it again.
If you are ollying off of a curb, or something smaller than just a few steps, you don't really need to ollie very high. You can, and it's great practice and looks good, but you don't need to. But any more than just a few steps (4 or 5), you'll want a good strong ollie.
After the pop, tuck your feet up and keep your shoulders square with your skateboard. Don't tilt or turn your shoulders - if you do, you'll spin a little in the air, and that will hurt more than ollying too late.
You want your feet tucked up so that you will be in the air longer. Shove your knees into your face, and get some good high hang time.