Squash is one of the most intense and physically demanding sports in the world. In fact, Forbes magazine rated it the “most fit” sport. While success on the squash court is also determined by strategy, skill, and mental toughness, one will not fulfill their full potential unless they are in top physical shape.
Eat healthier. This does not mean eat less, and this is a common misconception of diets for athletes. You simply need to eat better. Carbs are good before exercise, but otherwise are stored as fat. The opposite goes for protein (protein is stored as fat unless it is helping muscles recover after exercise). These are just the basics, and there is much more you need to be informed of. You should basically just be knowledgeable of what you are putting into your body. Think of food for people like gas for cars: you can fill a car up with whatever type of gas you want, but if you want peak performance, you will fill it up with the best gas you can.
Do Plyometrics. Plyometrics are becoming a staple in all modern sports. In short, plyos are basically short, explosive burst exercises. Squash is a sport, especially now a days, that requires you push hard off the t as fast as you can repeatedly. Therefore, plyos are needed. Good exercises include short sprints, jumps, etc.
Get strong. While there are differing philosophies on what build is ideal for squash, one thing is certain: the faster and stronger you are, the better. In terms of upper body, you need to generate enough racket head speed to flick the ball with pace. This can be done through either exceptional technique or making your muscles stronger (which can be done through various ways, such as weight lifting or hours of hitting the ball hard). As far as lower body goes, biking and weightlifting help tons, along with the plyos as stated earlier. The key thing to remember is not to get too much muscle mass. If you become too big, you will become slow. The key is to keep your body strong in relation to it's size. For example, I know people who are 140 pounds but have very lean muscle, they are extremely mobile. On the other hand, I know people as heavy as 190 who can still storm around the court because they have ridiculous leg strength to push off with.
Do ghosting. Ghosting is a technique developed in which you are on court by yourself and practicing moving from the t to the 6 hotspots of the courts: the front left and right, the side left and right, and the back left and right. It is important that your movement is good so that you can be efficient and save energy as you are playing. Some people are so efficient that as a result, they can sacrifice a bit of fitness.
Play matches! Practicing moving and playing in real situations will only make you harder, better, faster, stronger!