As we've said before, it seems that the serve often takes a backseat to the rest of the game. Everybody who knows about tennis believes that your serve is the most important weapon but how many players, in fact, practice what they preach? Not many. During "lessons", serves are usually struck at the end (if you're lucky, the last 10 minutes; more likely, the last 5) and it's almost as an after-thought. Maybe this gives the chance for the coach to relax or to be up-close-and personal with the student. Parents also see serves as a "sedentary" position and, hey, since they are the ones footing the bills they want to see little Johnny sweating and not "standing around." Of course, the coaches do say that the serve is something that player can practice on her own but does she take him up on the offer?! Don't worry; I already know the answer.
When dealing with limited time, it is advisable for players and coaches to incorporate serves throughout the practice and, also, to have at least one day per week dedicated to hitting serves. For example, when a student has a problem with the serve - perhaps it breaks down during the match - the team (coach/player) can consider including the serves in all the drills. For example, are you doing figure-8s or cross courts? Great; have the player start the point off with the serve. Are you working on volleys? Have the player serve and volley. Same with side to sides and every other drill that you have. Through this, the player will not only get a lot of practice hitting serves (thereby building endurance) but they will also do so while in the process of being exhausted. However, the player is tricked into hitting a lot of serves and developing the stroke. This is often a good way to practice for women tennis players since, sometimes, they tend to not like hitting serves (sorry for the generalization).
Lastly, have a day during the week that is made up of mostly serves (maybe this is the day when the player also plays sets). Practice hitting serves from everywhere around the baseline and even behind the baseline. Set up targets all over the box and aim for them. Use a radar gun. Combine serves with sprints (so that the player has to concentrate on the serve when exhausted). Use a different racket for every serve (to get used to serving a second serve in the event the string broke on the first). Put a basket of balls at the service line and have the player serve and "rush in" to pick up a ball (simulating a serve-and volley). Combine the serve with a low hand-fed ball (to simulate a quick return from the opponent). Have a kick-serve competition (see who can hit the ball the highest/most angle). See who can hit the most second-serves in a row. These last drills are obviously great in split-lesson formats.
Develop your serve as a weapon and not only will you take pressure off yourself, but now you can use that added energy to put pressure on the opponent's serve during the match.