Just my .02 but here goes:
The H:L limit screws do two things. The L limit makes sure the chain doesn't fall to the inside of the bottom bracket when shifting to the "small" ring. The H limit stops the chain from going off the other side.
You can only adjust the L for effect when you are in the "small" ring. When you loosen the screw, you should see the derailleur move. Same thing applies for the H screw. You have to be in the "big" ring.
One of the easier ways to get things semi right is to adjust the barrel's a little bit to give you some playing room. When you get the H:L screws set, you can fine tune with the barrel adjusters.
If you have finite patience, I would not recommend adjusting derailleurs as it can be very tedious and frustruating. If you have OCD or are anal (like me), you will drive yourself nuts tryingto get it perfect.
And as the general rule, and as was said, you should avoid small/small and large/large combos as much as possible.