Every get-together of my extended family is not without a reference to the storied BBQ chicken my grandfather used to make. Turns out the story is a bit of a fish tale and over the years they have come to admit that it wasn't really the best BBQ chicken, often dry and overcooked, but it was more the memory of the family dinnertime that sparked the nostalgia of "Dad's BBQ Chicken." Family history aside, I have never been very impressed at my own BBQ chicken--it is never flavorful enough, often over or undercooked, and generally just "OK." Never again. I have discovered the two great secrets of excellent BBQ chicken, and really, it applies to roast/baked chicken and other cuts of meat as well. And it's not exactly a secret, just one of those techniques that I never felt was necessary (I am admittedly a very lazy cook!). The main secret is: BRINING.
Brining the meat, as you can read the details of here, is a process that infuses the meat with flavor and helps it retain natural (and added) juices during cooking. Like a marinade, you can add any number of desired flavorings to the brine solution, such as herbs and spices, fruits or vegetables, and other seasonings such as liquid smoke or chili-garlic sauce. The possibilities are endless, but you can also opt for a plain and simple sugar-salt solution. The linked page above gives you the techniques, so I won't reiterate them here, but only emphasize how beneficial it is to follow this process to producing a high-quality slab of grilled meat. Once your chicken has been brined, you can slather on a desired BBQ (or other) sauce if desired before you grill it.
The other lesson I'm quickly learning about BBQ is that a low, low temperature is essential when cooking chicken on the grill. I have a thermometer on my grill, and shoot for around 250-300 degrees Farenheit. Too high of heat obviously burns the outside of the chicken before the inside is done, especially if you are cooking on-the-bone chicken.
I hope these tips help! Happy Grilling!