Keep the bar a little way down your back. Its surprising how far down you have to get it to get comfortable, so practice with a light weight until you feel the right place. It should be just below your traps and in the middle of your rear delts. This position will also allow you to balance better whilst in the squat and keep you upright as you come up.
You have to squat down so that the top of your leg is below parallell to the floor. This is difficult to see, so in effect it is usually taken as the point where the crease of your leg at your hip is lower than your knee. Get someone to watch you as you squat to make sure you are below this point until you get used to it. It is always better to be a little lower than necessary in competition. Its not good to get back up, only to be failed on depth.
As you squat down keep your chest up. Look at a point high up in front of you if necessary. If you look down you have probably lost it.
If you have trouble balancing try moving your foot position around a bit. If your heels come off the ground whilst squatting, you dont need a block under your heels or raised up shoes, a slightly wider foot position and moving your toes so that they point out a bit will probably solve the problem.
Wide grips are usually the strongest, but the rules say that you must keep at least one finger on or inside the line scribed on the bar. If you have had a shoulder injury, such as rotator cuff damage you might have to bench press a lot narrower. As you recover you will be able to go out wider again.
You have to keep your bum and your shoulders on the bench, but you can arch your back off, between them, as much as you like. This changes the angle of the lift into much more of a decline bench press. This is usually a stronger position, allowing you to lift more. So work on your back strength and flexibility. Once you have started the lift you mustnt move around and your feet should stay flat on the floor.
The rules say that you must momentarily pause the bar on your chest before lifting it back up. The length of the pause is undefined. This is the most common technical reason to fail a bench press so make sure you practice pausing in training. It does make the lift a lot harder so get used to it.
Getting the breathing right can play a big part in benching. Take the biggest breath in that you can during the first half of the descent of the bar and hold it until you have nearly got back up to the top. It you let it our too soon you will lose the tension in your chest.
If you use an over and under grip and some chalk you are unlikely to let the bar slip out of your hands. It is usually better to approach the bar and grip it before crouching down and when you are ready crouch down and lift the bar immediately, so you are down for the shortest time possible. Lift with your chest up, again looking at a point quite high up will keep you in the right position. Try to do as much of the work as possible with your legs, they are much stronger than your back. When you get to the top you have to stand errect until the referee tells you to put the bar down. The rules say that you have to let the bar back down under control. This usually means still having your hands on the bar when it reaches the ground. In the gym on the other hand it means putting it down without it crashing to the ground. We dont want to scare other gym users or damage the equipment !
In competition you have to wear a one-piece unitard and whilst bench-pressing a short sleeved t-shirt with it. Shorts and vest etc are not allowed. The reasoning behind this is that the referees can see where your clothing is without it hiding your body.
It is best to wear flat stable shoes in training and competition. Normal running shoes are fine to begin with but as the weights get heavier the soles can compress and you lose stability, especially whilst squatting. So something with a thin hard sole is best.
The best belts are wide up to six inches. In competition you are not allowed padded lumber supports in them, so check yours. You can buy superb expensive lever action belts, but they dont help you lift any more, they are just easier and quicker to get in and out of. So stick with something simple to begin with.
You are not allowed to wear any advertising whilst you are on the lifting platform, apart from the name of the event that you are attending, the british flag, the name of your lifting division or your club t-shirt (thats us). So be careful with things like names on your belt. It is ok to cover them up with black tape or paint them over.
Other kit that you are allowed is elasticated knee wraps. They have to be regulation length (2 metres or under), be careful because as they get older they can stretch and go over length. And wrist wraps, these are like knee wraps only narrower and shorter. These can be useful when bench-pressing but also when squatting. Give them a try and see what you think.
As you progress you might want to try lifting suits and benching shirts. These are very tightly fitting and only slightly elasticated garments. They act in the same way as belts and help to spread the work over more muscles. They allow you to lift a little more weight, but also make things a little safer. These are available from a very few outlets, probably no more that 3 or 4 places in the country and have to fit exactly to help much. They are also extremely uncomfortable to use.