Don't use extra lean beef for your hamburgers. You'll need at least 15-20% fat in your beef so it will stay moist as well as have good flavor. You can also make a seasoned compound butter by mixing butter with salt, pepper, minced garlic, minced herbs, and/or lemon zest. Chill the butter, then encase 1 tablespoon of the herbed butter in each hamburger patty. Grill to 160° to kill any potential bacterial contamination, but higher than that will make your hamburgers dry.