Movement in squash is probably the most important aspect of the game. It is also one the things that really separates different levels of players. When moving to the ball there are a few key elements to consider. Most of the time if you can you need to get on the ball quickly to have the option of putting time pressure on your opponent. You also need to approach the ball in a manner that leaves you options as to where you hit the ball. One of the most critical points of moving to the ball though is leaving yourself in a good position to move off of the ball. I believe this is probably the biggest differences between high level players and lower level players.
The first part of moving to the ball quickly is nothing to do with moving is to do with watching. You need to watch your opponent carefully to be able to determine quickly where they are hitting their shot. If they are behind you it is a good idea to try and leave your feet fairly square to the front wall and turn your shoulders and head rather turning your whole body. Once you open up your body too much to the side your opponent is on it makes it more difficult to move to a ball that is hit cross court. When you start to move you will usually start with a split step and taking a powerful first step will get you to the ball quickly to be able to apply time pressure to your opponent.
The line you take to the ball is also critical. You need to get to the ball quickly but you also need to take a line that allows you choices when you arrive at the ball. If you take a line that leaves you to close to the ball you will have a hard time hitting it straight. If your opponent is observant they will see that and they will realize that you can't likely hit it straight thus making it easier for them to figure out where you are going to hit. When moving in to the front court approaching the ball on a diagonal line will leave you the choice of hitting a straight drive, a cross court drive, a straight drop or flicking the ball cross court. The line you took to the ball gave you those options and it really makes your opponents life more difficult as they have to be able to cover all 4 corners. When moving in to the back corners you need to ensure that you get behind the ball to some extent so that you can get some weight transfer in to the ball or are able to hit a cross court. In the deep backhand in particular if the ball get behind you it is very difficult to hit a cross court shot.
Your body position when you are hitting your shot needs to be stable and at the moment of contact there should be little movement. If you ever want to find a professional player to model for this look no further than Amr Shabana. He is incredibly still and stable through contact with the ball. The position of your feet and where your weight is though is very important to your ability to move off of the ball and back to the 'T'. Often when under pressure and late to the ball the lower level player will still be going forward when they hit the ball and not be in a position to stop and move back to the 'T' quickly. Higher level players will use the lunge to get to the ball and stop their forward momentum before they hit the shot. That also leaves them in a position to push back to the 'T' to be able to cover the next shot more effectively.
Moving off the ball is critical. Your shot selection should give you some time to recover the 'T'. It is fairly common to see players try to blast their way out of one of the back corners. If the ball is not tight enough though and your opponent takes the ball on the volley you are taking time away from yourself. This makes your life difficult not your opponents. Worry more about using height or keeping the ball tight so your opponent will not be able to volley. A high soft shot for example will hopefully get past your opponent and even if it does not you have given yourself time to get back to the 'T'. The front corners can be particularly difficult for some players, especially if they are counter dropping from there. Moving back out of the front corner and getting as close to the 'T' as you can is imperative. You need to move back until your opponent is ready to hit, let him hit and then move. You are of course looking to try and figure out what they are going to hit but you can't guess. The idea is to move back far enough to be able to cover the back corners yet not so far that you can't cover a counter drop.
Movement to and from the ball is one of the biggest difference between pros, top level club players and beginners to the game. Generally speaking the better the player the better they are watching their opponent to figure out what they are going to hit. They take a better path to the ball leaving themselves in a position to hit a variety of shots. They also leave themselves in position when they are hitting so that they are can move effectively off the ball back to the 'T'. If you want to take your game to the next level work on taking your movement to the next level. Study a player that is better than you that moves very well and model their movement. Not only will you get more balls back you might well also hit better quality shots.