I have devised a new foolproof system to relic guitars. It takes between 2 weeks and 6 years and works on both solid bodies and acoustics. The results are always unique and require little in the way of tools, but does require a few friends and a few common vices.
Step one: Give guitar to a small child and let them drag it around the house for about 2 weeks. This will simulate years of bump damage and introduce a variety of swirls and scratches. If the child is artistic, randomly place a Sharpie nearby and leave the room for about 20 minutes. You should return to find some neat patterns drawn on the guitar. (You may find the designs not to your liking, if this is the case try removing them with some goo-gone or similar, it will help damage the finish and let the sharpie bleed into the wood)
Step two: Take up smoking and buy a few cartons. You may already smoke so might have some super kings to hand already. When playing the guitar be sure to always chain smoke, this will stain things yellow and add the rich smell of tobacco to the guitar. Play the guitar at least 3 hours per day and take it out and busk on a street corner on weekends. (This step can be done over many years of playing and could allow you to skip all the other steps)
Step three: Buy some cases of beer and invite friends over. Do this weekly (daily if you can manage it) and play guitar while drinking beer. Allow drunk friends to also play the guitar and be sure to spill at least 2 beers per session in the sound hole. Have a lot of fun with this one. Take it on camping trips, play it near open fires, etc.
Step four: Arrange for a friend or relative to "steal the guitar" when you go away on holiday. Let them know you expect them to remove or damage the serial number and maybe attempt a poor refinish with a can of cheap spray paint. Much like the child in step one, a friend who is artistic will really do a nice job here. When they are done tell them to take it to a local pawn shop.
Step five: Buy the guitar back from local pawn shop for $50 bucks. (Someone might buy it before you, so you may need to skim craigslist or ebay for a bit until it pops back up again)
Step six: Visit a guitar forum and ask for tips on how to strip and refinish guitar you just scored at the pawn shop as the finish looks warn and the guitar smells like beer and cigarettes.
This system is entirely cyclical and can be repeated multiple times until desired effect is achieved.