The secret to a perfect grilled chicken sandwich starts with the chicken. To keep it good and lean use skinless, boneless chicken breast with any loose fat trimmed off. Pound the breast to a uniform thickness, about 1 inch. Then cut it to size. If you good with a kitchen mallet you can shape the breast as you pound it. Now you have a choice. You can brine, marinade or rub the chicken. A brine will super moisten the chicken and keep it tender while grilling. Marinades help hold in the moisture and add lots of flavor. A rub will flavor the chicken, but it can dry out fast so I would suggest brining first then applying a rub.
Brine on small chicken breast pieces can be done in as little time as 30 minutes. In fact I would suggest not brining for more than an hour. The chicken will not absorb more water, but it can pick up extra salt. Depending on the strength of your marinade you can do it in anywhere from 30 minutes to 6 hours or more. Don't let it sit too long though.
The next step is the grilling. Start with a good hot grill and keep a close eye on what you are cooking. If you have used a marinade containing oils or sugars it can cause the chicken to burn fast, so you might need to keep it moving to avoid burning. Remember, chicken is cooked when it has reached an internal temperature of 165 degrees F. The juices should run clear and the meat should have a even white color.
Now you have the makings for a great sandwich. Top with everything from mayonnaise to grill roasted bell peppers to mustard and pickles. Buy (or make) great breads and you have everything from the perfect half-time snack to the main course of a fancy meal.