Coming from someone just starting to 'hop' (and also keen to hear advice)
Do you jump hooked in?
Depending on the size of the jump, small-yes....big, ummmm never done a big jump!
Do you sheet out when going up the ramp?
NO - Sheet in and push your feet forward (got to get the max amount of air

)
Are you sheeted in in the air?
I find by being sheeted in, the landing's softer....coming from a novice chop-hopper, I feel more in control when sheeted in - keep body position tight.
Overhand or underhand front grip for jumping?
Since starting to go underhand, my technique's improved heaps.
What is the best board angle to land?
For starters - tail first with nose around 45degrees downwind for quick recovery (again...I'm a novice!)
Any good websites to tips?
Peter Hart's got a good video "Introduction to wave sailing" (or something like that, shows a couple of good starting pointers, although watching someone with his ability doing what he calls 'basics' is a bit sickening for us beginners)
Hopefully someone that has more experience will give some valuable advice, the above is just from my experiences, and they're pretty limited!