Some of the best tips for grilling boneless chicken thighs are to marinade the chicken prior to cooking it and to coat it with a small amount of oil before placing it on the grill to prevent sticking. Keeping the grill, whether charcoal or gas, at an even medium heat can help to cook the chicken through without overly charring the outside. It can also be helpful to limit flipping or moving the meat unless you are using a sauce.
Although thighs are often considered more flavorful than their white meat counterparts, grilling boneless chicken thighs tends to dry out the meat more quickly than other cooking methods, even when done properly. Marinating the chicken prior to grilling it can help to maintain its texture and moisture, as well as add additional flavor. Even if you want a sauce or spices to be the prime flavors in the chicken dish, letting boneless thighs soak in a simple mixture of oil and vinegar or citrus juice can help the chicken to maintain its moisture while still leaving it bland enough to stand up to sauces or rubs.
Boneless chicken thighs, especially those with the skin removed, can often stick to grill grates, making it difficult to flip over the meat without tearing it. Applying a light coating of oil with a high smoking point, such as canola, vegetable, or peanut oil, can help to prevent the chicken from sticking without burning the outside of the meat. Oftentimes, if you use a marinade without oil in it prior to grilling boneless chicken thighs, this will be enough to keep the meat from sticking.
Grilling boneless chicken thighs at too low or too high of a heat can cause the outside of the meat to either not brown or become overly charred. In general, this type of chicken is best cooked over medium heat. With gas or propane grills, you can simply set the grill to the proper temperature with the dial. In general, holding your hand over a charcoal grill can give you a good idea of how hot it is: if you can comfortably leave your hand over the grate without touching it for four to five seconds, then the grill is at medium heat.
Over-flipping is one of the most common mistakes made when grilling boneless chicken thighs. The chicken should only be flipped one to three times during the cooking process, unless you plan to apply several layers of sauce; in this case, the sauce should be applied towards the end of the cooking process, and the meat can be flipped over several times for an even coating. The thighs should be placed on the grill and allowed to cook until the underside is browned and the meat is easy to move off of the grate. Once it is flipped over, it can typically be left alone until the meat is cooked through, provided that the grill is at the proper temperature. Thicker thighs may need to be cooked on each side twice to avoid over-browning or charring the meat.