While I shoot mostly American football, my feelings about shooting both of these sports is similar. Most of these are personal preference, but I think you might find something helpful.
1. Shoot from field level. Obviously this is not an issue if you're shooting smaller leagues that play outdoors with no real stands, but I find it's important. In my opinion, the higher up a shot is taken, the more it looks like just a fan snapshot than a professional photo, no matter how good it turns out. Again, my opinion.
2. Shooting from midfield is not as good in practice as it seems in theory. Sure, it sounds awesome to be right in the middle, but you're really making it more difficult to get good shots of where the scoring (and the most action) happens. There are a LOT of bodies on the field during most team sports, and they'll get in the way at the worst times. Not to mention that this is where the team/parents/fans set up camp during the games mostly. I try to shoot from behind or close to the goal line in American football and between the 6 and 18 in soccer (occasionally behind the goal line, but rarely). You'll miss some action at the other end, but in my opinion a great shot down at the close end is better than getting a decent shot at both ends from mid field.
3. Try to cover as much focal range as possible. Whether this means carrying 2-3 body/lens combos or using 1 body with something like a 28-300 (beautiful for outdoor sports,) I feel this is very important. A 70-200 will be a good range for most things, but having a quality wide-angle can come in real handy.
4. Learn as much about the sport itself as possible. I don't know your level of soccer knowledge so I don't mean this to be offensive if you know a lot, but I find it more important than anything. If you know the sport well, you'll be able to anticipate action well. If you can anticipate the action, you will get much better shots. As ridiculous as it sounds, before I started working for the indoor American football team I work for, I spent a lot of time watching old indoor/arena football games. It really helped me get it down and know what to expect. I'm really glad I did, too, because while I know football, indoor football is a whole different game. Much like watching how World Cup soccer is considerably different than a league of 13 year olds lol.
5. Shooting modes: Fast shutter speeds (I like to use at least 1/500.) Continuous focus mode, more open apertures (f/2.8-5.6) for subject isolation. I personally recommend using Shutter Priority or Manual Mode with auto ISO. I feel this always gives me the best results. Again, you may have something else you're more comfortable with.
6. Know your background. I know that shooting sports, there will almost always be SOMETHING in the background, but I always try to find a spot that will give me the best chances of having minimal possible distractions in the shot. That's one benefit of shooting from a higher vantage point, but I still don't like doing it.
I know a lot of these are personal opinions and might not be what you're looking for, but I hope at least something in my ramblings will be useful to you. The most important thing is always to just practice as much as possible.