Snibble has given you no dribble!
He hit it on the head, squarely and firmly. Research the subject you are modeling, see what conditions it operated under and what it may have been subjected to. Take into account things such as natural weathering, maintenance, combat damage, and plain-'ole heavy usage. Here's a short list of some of the "usual suspects":
Exhaust staining
Oil / hydraulic streaks
Faded and / or stained paint
Chipped, scuffed and generally worn paint
Dirt, dust, grime (thrown-up dirt, sedimentation from rain, or greasy grimy hand prints from ground crews and such)
Replacement or repair of damaged parts or panels resulting in differing shades of the "same color" paint
Techniques are another matter, and there are many of them. But you can't apply a technique if you haven't a clue as to the end to which you're applying it. Research, research, research!
Fade to Black...