I already know why you're probably thinking..serve, return of serve? That's pretty basic and that isn't an issue in my game. I guarantee I can improve both with two simple rules. The first is when you're serving, after you serve, keep the T...force your opponent to hit a high quality shot to get you to the back corner. Most people give up the T on average and even subpar return of serves. My second rule is for returning, and yes, you could probably guess, get your opponent off the T and into the back (* although I should mention her that I saw Mohamed Abouelghar play a couple of years ago in a PSA event in Vancouver and he didn't hit a straight length on a single return of serve. Perhaps he was working on attacking on his serve as he was only about 19 at the time or maybe he found some weaknesses agains his opponents because they were not accustomed or comfortable being attacked off the serve. He had a lot of success with this and made very few unforced errors on his returns). So let's assume you don't have Mohamed's racquet skills and you're like most mortals that are better off hitting most return of serves deep. If the server is not trying to volley your return of serve than you are lucky..try this against someone that serves well and is aggressively trying to keep the T it is a lot harder.
This is an easy thing to practice and will improve game because of the way you are setting up your rallies. You can do this in a regular game, a length game, or in one of my favourite games where you race to x number of volleys and fight for the T. I also like playing a 2-opposite corner game. For example, the back left and front right. In this game the returner cannot hit crosscourt length or straight drops on their returns. The server can also cheat a little when they serve well and can look to aggressively cut off their opponents return and forces them to hit a very accurate length. Of course the front right corner keeps both players honest and makes for some fun rallies. You can also have the same focus while playing the always popular 3 corner court.
You can also try a simple serve-return of serve game where all they get more points if they return into a specific target. This is a good one for kids and the server can try different serves and see which their opponents have the most difficulty with. In this routine there is only 2 shots, so the focus in more on creating a weaker return of serve. For advanced players you can mark points with tape, say 1 or 2 floorboards for 5 points, 5 or 6 for 2, and 0 for anywhere else. I normally say a return of serve is better overhit than under hit so I normally make them have to hit the ball behind the service box (unless they are more intermediate/beginner players). In this game the one player can serve for the entire game to the same side or you can rotate them around after each serve-return of serve. The first player to get x number of points wins; then switch sides.
I also want to mention that serve variety is hugely underrated in squash. Why do people serve right to where their opponent is holding their racquet I'll never know. Of course you can still hit a tight serve into the side wall which can make the decision of volleying before or after the side wall difficult for intermediate players. But at a higher level this becomes simple, even more so if you serve the same way game after game. Try varying the angle, pace, and height. At a high level of squash even good players will have trouble with this variety. Even if it only means their serve is a few inches more off the side wall or hit a couple of miles of hour slower this may be all you need to keep the T (as opposed to going to the back corner).
Here are some of the different serves that I've found to be most successful. I enjoy a hard serve right at or just behind a player when they are returning serve on their backhand side (even more so when they are fatigued). I also like a lob serve to the backhand. To the forehand side I find an overhead serve aiming to hit the nick or low on the side wall where they would normally volley it from causes a lot of players to back up and then have to boast (or even nick and ace). Yes, even at a good level of play you can set up more points on your serve and you can neutralize your opponent by getting them off the T with good service returns.