Alton Brown is the king of pop grilling. Some of his tips -

* put a cooling grate on your cooking grate to give you a tighter cooking surface area for cooking lower mass foods (small vegetables like asparagus or zucchini). This prevents them from falling through the grill.
* when grilling onions, take three skewers and ram them through two full onions. Now cut the onions into three parts, using the area between the skewers as a guide. Now you have three skewers with two giant onion slices on each. Grill each side for 2-3 minutes and you have perfectly cooked onion slices without any falling into the grill.
* for tomatoes, oil up the grill so it becomes like a "non-stick" surface. Then cut the tomatoes in half. Sear the tomatoes on the cut sides for a few minutes, then turn them around on the skin side and put oil/garlic, salt, and some basil in the hollow areas of the tomatoes. Leave it in for a few minutes and you have perfect tomatoes to use how you see fit.
* For fish - oil up your fillet! Then apply a rub of your creation or pre-made, which gives you flavor and a sort of non-stick surface. Your grill needs to be hot for fish, and you need to oil the grill with a cloth right before you put the fish on it. Before turning the fillet, oil up the side of the fillet you're looking at and when you turn it, spatulas suck - use a turkey lifter (or two if it is a large fillet) to just poke under and turn the fish over (assuming you left enough space for flipping it over). When it is done, use the turkey lifters to gently get the fish off the grill and serve it.

Don't overcook steak as it becomes tough and don't undercook chicken as it is dangerous.

If you are planning for outdoor grilling, then there are 2 types of grilling to consider such as direct and indirect grilling. Direct grilling is best for foods which are easy to cook such as fillets, fish, hamburgers, steaks, boneless chicken and hotdogs. You have to make sure that you put the meats or vegetables directly over coals and that the foods will be cooked in less than half an hour. If you are going with the indirect grilling technique, you gotta make sure that you place the food on a drip pan before putting it over grill. Also, to improve the taste of the food, you can pour some apple juice, pineapple juice or other flavored liquids in a drip pan while grilling.

Hope this helps.

source : http://www.deartips.com/outdoor-grilling-tips.html