Although the serve has many moving parts leading to a breakdown of the serve during the match, the mistake can usually be attributed to one of three components: (a) TOSS; (b) BALANCE; and (c) LEG DRIVE. Besides practicing inordinate amounts of serve, here are four tricks that you can use to solidify your grasp of these concepts.
1. Use a half-filled water bottle to practice tosses. Unless you have a calm toss, the water inside the bottle will cause it to wobble and move around. Instead of "tossing" the ball/bottle, practice putting it up in the air so that you can catch it in the same spot where you release it.
2. Toss the ball against the fence and "catch" it between the racket and the fence. For training the proper contact point and proper arm extension, stand facing the back fence (about 2 feet away) and toss the ball slightly forward into the fence. Swing the racket towards the ball and see if you can trap the ball between the string bed and fence at full arm extension.
3. Use "leg-cuffs" for teaching your hind foot to lock in position alongside the front foot. A lot of players who bring their back-foot forward (as opposed to serving off both feet - e.g. Federer) tend to overshoot the stopping point causing their hind foot to go in front of the front foot. This causes the hips (and, consequently, the chest) to rotate prematurely. Train your hind foot to "lock" into the proper position by using elastic/rubber leg-cuffs (generally used for speed work) which pull the feet close together without allowing the hind foot to swing forward out of control.
EXAMPLE: Elena Bovina demonstrates the bottle toss and leg-cuff practice.
4. Use a 2" x 4" piece of wood to train leg power. Place the piece of wood on top of the baseline and try jumping over it as ou serve. Proper knee bend and explosiveness will cause you to propel over the obstacle and land into the court. If you clip the obstacle with your toes it means your serve is lacking leg drive and, most likely, that you are "arming" the serve. Distribute power along all muscle groups from the ground up and your serve will maintain "pop" throughout the match. A lot of players start out the match ripping the serve with only their shoulder muscles. However, these muscles are normally very small and tire easily. Accordingly, the serve will invariably start breaking down as the match progresses. By recruiting more muscle groups for this stroke, you will be able to maintain a proper stroke for a longer period of time