In general terms with the 300mm 2.8 on a 1.3 crop you can cover about half the pitch and still get reasonable resolution out of a crop. When the action gets frame filling close then flick into portrait mode. So if you sit at the half way line or thereabouts you can cover most of the match.
However, I rarely do that (sit at the half way line). First thing I look at is the light direction and then the wind direction. Ideally I want the light on the faces of the players so I will try to get the light behind me. I then want the wind behind me as well if I can or side on if I must. This reduces the amount of dust, rain etc. coming into the lens. Check the corner flags for wind direction. Look at the wear marks on the sidelines and this will give you the lineman's run. Avoid shooting from behind the lineman if you can. The nature of his job is such that he will be between you and the action a fair amount of the time.
A good position is halfway between the goal and the corner post. You will only get attacking action from one side but a significant part of the action will be coming towards you and it is always good for 'seconds' from an opposite end goalies kick.
The other good position is halfway between the goal line and the half way line. You can usually cover a fair amount of pitch from this position and are in a good position for jumping headers from goalie kicks.
Get some manager shots if you can. These can add a different perspective to your coverage. If you miss a goal (which you will) try to get the player as he turns from the goal to celebrate.
Low is good as Kipax says. Avoid standing if you can. I have my kit in a large toolbox. It allows me to sit quite low to the ground without actually being on the ground. I have got very low on occasions but find it difficult to move when a ball is belted towards you (went 3 seasons without getting hit once and then got hit in two consecutive matches). A monopod is useful/essential if you are low. I tend to be in portrait mode most of the time as distant action is going to be cropped into anyhow.
I am a club photographer so my coverage is aimed at the team I am covering and that also has an influence on my positioning.
At about half time you will be losing the light this time of year so be prepared to get to 2.8 and ISO 1600/3200 with shutter speeds of 1/320 (if you are lucky). Non-league grounds are not noted for their great lights
Enjoy the day and make sure the secretary gives you a cup of tea as well
PS. An example of the kind of coverage I get from the low height middle of one half position is
John
