Well, the "most important" tip is a tall order. It depends a lot on what type of grill you have and what you like to cook, but here's a really important, general tip: Give yourself at least two heat zones. If you set up your grill for one type of heat only, your options are limited. What if something is cooking faster than you want? What if your food is flaring up? What if you grilling two very different foods at the same time? You should have at least two zones of heat: one for direct heat (where the fire is right under the food) and one for indirect heat (where the fire is off to the side). That way, you can move your food from one zone whenever you like.
By the way, that one of my "Top Ten Tips for Grilling Greatness" in Weber's Greatest Hits. In the book I also have the Top Five Tips for Grilling Like and Expert.