The key to ranking up fast in Call of Duty: Black Ops II lies in two areas:

1). The ability to kill people fast and frequently.  This doesn't necessarily mean you have to kill them more than they kill you.  It just means you need to kill someone on a regular and quick basis.  To do so, just sprint towards where the action is, but take care to be aware of your surroundings.  If you are walking down a long area with no cover to the sides, aim down your sights.  If you have cover, be aware of where it is and use it when needed.  Use the radar if you can.

This point is really more about playing style than anything else.  Don't camp with an assault rifle or sniper or hide in a corner. Move towards the action.  Use a weapon that is good for short range, probably the SMG, unless you are good enough to use the AR or Snipers at short range.  The shotgun is okay, but it will be worthless at medium ranges.  

2). Your choice of killstreaks.  If you aren't able to get to the 5-6 kill streaks on a regular basis, just use the lower ones.  Getting the lower killstreaks on a regular basis will net you more points than always trying to get to 10+ killstreaks, yet rarely reaching them.

You also might consider using the perk that gets you killstreaks faster.