It's generally accepted that a reduction of heroin intake will result in an increase in peripheral vision.
Lol, no seriously though IMO the best advice is that already offered, you don't need to move your gun when you look around, always keep an eye on both sides of the field and the center.
What I used to do, takes a bit of practice but is useful when learned, try and make a mental picture of the field from above from walking it, then take a note of which opposing player is at which bunker, then you don't even have to look for the players quickly pop out and check a bunker and if hes not there he's moved..
That was a really shaky explination lol.. its kinda hard to explain.. Do you sort of get what I mean?