The best possible serve is the one that finds the sidewall high with a mid pace so that your opponent has to scrape his return off the wall as otherwise the next bounce on the floor would die around the backwall nick. If you can do this with a high reliability percentage, your opponent will take a position more and more towards the sidewall whist waiting for your serve; this is when you can start to serve occasionally to his body as well.
Serving to the body requires a slightly higher pace, first bounce should be just a few inches above the service line and you should aim to find your opponent's body around hip heights. It is very difficult to aim for a dying ball in the backwall nick as for that the ball would be slow enough to allow your opponent to get out of the way and play a reverse angle shot (forehand from the backhand side and vice-versa) - unless he is tired/lazy (for example after a long rally that you have won) to do so.
Anyway, you have to make sure one thing with the 'to-the-body-serve': don't hit it above your opponent's head, as Daryl Selby is doing it in the below rally. Guys with quick hands will punish you straight away as Ramy Ashour demonstrates it here, whereas slower-handed guys will get an opportunity to step away and wait for the rebounce off the backwall.
If you have a subscription with squashtv, then go for some Nick Matthew match replays and observe how and when he is employing this type of serve; he is by far the most efficient with it (James coming second in my opinion, whereas in former times John White was a frequent and efficient user of it).