I used to play it all the time (I assume your on the PC)
Start off using the Assault model; it's an all rounder and has decent armour), and all I can recommend is practice like buggary.
Other methods of improving your rank are to become the gunner in a helicopter while using the mechanic model (as you'll automatically repair damage the craft picks up), or learn to use a buggy and steal bases regularly on the special forces kit (making use of the sniper/assult weapon and have plenty of sprint ability.)
Otherwise, the only advice I can give is practice like buggery. You'll need it.
