A few tips:
1) An accurate meat thermometer is a must. This will allow you to check that thick slices of meat or chicken are safely cooked all the way through. Make sure it is the instant-read kind so you can just poke it in and check it.
2) You can generally tell by the firmness of meat and chicken how thoroughly it is cooked. The firmer it is, the more well done it is. You can use your (clean) finger, as long as you have washed your hands since touching raw meat.
3) Only slabs of meat can be done rare. Ground meat or any type of poultry should be thoroughly done and not have any pinkness to it.
5) Low heat cooks more evenly and helps prevent an overdone outside / raw inside problem. One issue with low heat for a longer time is that it can dry out the meat. A spritz of oil (such as olive) and/or wrapping the meat can help keep it juicy.
7) Always leave the skin on the chicken if possible, to keep it juicy.
8) If a small area of the grill is covered with aluminum foil, you can put food that is beginning to singe or cook unevenly upon it to slow down and even out the cooking, and reduce burning.
10) Fatty ground beef makes great burgers. Just make sure to keep an eye out for flare-ups.
12) Marinades are usually applied anywhere from a half-hour before grilling to overnight the night before. Rubs and seasonings are applied right before grilling. Sauces are applied about 10 minutes before the meat is done.