LQ
It really all depends on your airbrush skill. Do you have a dual action brush? Single action brushes need fine needles and tips for outline work, and usually you need to buy these separately.
I have run pencil outlines as a guide on a camo jobs and it worked okay, but I have had an occasional overspray problem with the airbrush (too little air I think). I use an Aztek A4709 which comes with different nozzles. I use the finest (Tan .40) but after talking to a couple of guys in my club, the recommend the next size up (Black .50) as they say they had the same overspray probs with the Tan nozzle. To date, I have used masks just to be on the safe side.
The simplest mask is Blu-tack and paper. You place a thin 'sausage' of Blu-tack as the edge of the camo. Push a bit more in behind it, over the area not to be painted and then push a piece of roughly shaped paper into that to stop overspray. Spraying up to the Blu-tack will give you a hard line (don't spray too much paint or you will end up with a pronounced 'lip' between colours); Weathering will soften the edge a bit (this is why you don't want the lip of paint - the lip will show up horribly if you dry brush a different colour over it)
For a softer line, shape the paper to the camo design and using Blu-tack again, push the paper onto the model. Leave the edge of the paper to hang over the blu-tack,a few mil above the model surface, and as long as you spray flush to the paper (not at an angle under the edge) you will get a soft demarcation line.
For straight edges, like on the Swedish S-Tank, use Masking Tape as Dwight recommends. I get proper Painters tape (it's green or blue over here and is about 3 times the price of cheap masking tape) as this is low tack, but will not stick to the model if you leave it on for more than 24 hours. A problem that standard Masking Tape can create. Always remember, you get what you pay for...the cheapie shops sell tape for almost nothing, but expect it to bleed paint, fall off, react with the paint, etc.