There are a few things to keep in mind when trying to get really tender, juicy, grilled chicken breasts. Tenderizing your chicken will really make a big difference when grilling your chicken. Don't over-pound your meat - a few good whacks will do the trick. Your chicken breast will cook a lot more evenly if the meat is a uniform thickness.  Marinating your chicken is also a great way to keep it moist. The high PH from the vinegar or lemon juice helps break down the muscle fibers in the chicken and the oil will help keep it moist.  Last but not least, don't overcook your chicken.  The best way to assure you're not overcooking your chicken is to use a digital meat thermometer.  Grill the chicken only until it reaches 165 degrees.