Grilling burgers: flip once, or keep flipping?
I'm solidly in the flip as often as you want camp.  As long as it's not over (or under) cooked I find that you get better results flipping often. Plus you can rearrange things to make more room as you flip.

To tell how cooked a burger is poke it with a finger or something else that's not too sharp; just like a steak the more cooked it is the more firm it will be.  You just need to learn how "squishy" you like them.

Also, as fun as it is, don't press the whole burger into the grill!  All that happens is that the delicious juices are forced out.