Know your sport
. Know how the sport is played, know where the action should happen, know the key players, plan ahead.
For most sports, you want the action coming towards you. Want want clear visible faces. And for ball sports, you need that ball in the frame.
For rugby, it is usually best to sit behind the try line. Let the players run towards you. Hopefully too, you can capture a try or two.
For football, sitting near the corner flag is good. You get the players running towards you, and you can also get an angle on the goals.
Every sport is different. But the more you know the sport, the better you can anticipate the action, and get the shot you want. Swimming, running, cycling, horse racing, football, rugby, etc - know the sport and plan ahead for the action.
You can't get every shot. You need to be selective. Wait until the action comes to you. Where possible, fill the frame with your shot. More often than not, you will still crop in to the image, to focus the photo on the action. When it is not possible to fill your frame, use software later to crop in to your image. But, there is no point in shooting action at the other end of the pitch and then cropping so much that your end result is tiny.
Be prepared for all situations. Bring a chair, stool, step to sit on. They can be cheap and very handy to have. Bring rain gear. Even on nice summers days, a grass pitch can be damp. Hats, gloves, etc can also be useful.
Bring rain gear for your camera too. This can be as simple as a plastic bag, or as expensive as full raingear for your specific camera/lens. Either way, you will want to protect your camera.
Bring a drink and some food if needed.
If you don't know the venue well, get there early. Have a walk around. Find shelter, find food, find the toilets. Then find a good spot to take your photos from.
When possible, say hello to match officials. Introduce yourself to the coach/managers of the teams. If shooting for the press, get teamsheets.
With editing photos, do as little as possible. Try to get your exposure right. While players are warming up, you can get practice shots in. Adjust your settings as needed. Also watch for what is in the background
of your shots.
There are many different levels of sport, from children's sport all the way up to senior professional/international sports.
In general, the higher the level of the sport, the harder it is to gain access.
Most parents want to cover the sporting events of their children. They way to see their child perform and want to be able to show off how good they are. In the modern politically correct and "sensitive" time around children, photographing children's sports event can require a little preparation. Some clubs/organisations require you to be police vetted. At the very least you should approach the club and ask permission to photograph their games. You should then also speak to the coach/manager at the start of a game. Just introduce yourself, and tell them why you are there. After that, in most cases, there will be no issues. Of course, there can be instances when some parents don't want their child's image used. Respecting their wishes can go a long way to helping. No matter what, always be polite - it will get you a lot further.
Most areas have some local sporting events. Local public pitches, local sports clubs and societies. This tends to be the easiest level to gain access to photograph. Usually you can just turn up, ask permission of the team manager(s) and then shoot. Local sport tends to be more emotive. Plenty of crunch tackles, great facial expressions and such. Well worth covering. Talking to the local clubs can build up good relationships and can also be a step to gaining further accreditation in higher leagues.
Shooting for the press or magazine or having proper accreditation tends to be the only way to get access to these events. But, shooting these can be very rewarding (photographically speaking). You tend to get cleaner action to photograph. You also shoot in better stadia, which provides a better background for images. A full stand in the background looks better than an empty one.
Generally, the higher the levels of the sport, the easier it is to capture good clean images. Children tend to play swarm sports (where the whole team chases the ball), while the professional sports tend to have cleaner images with just 1-2 players involved. But, that shouldn't stop you getting great images, no matter what you cover.