Maybe you're in a post-apocalyptic wasteland. Perhaps you just crawled through a sewer. Or you may have just gone head to head with a Stormtrooper and came out of it with a bit of blaster scoring (Those rumors about their inability to hit are really exaggerated). Whatever the reason, sometimes dirtying up your pieces makes all the difference in selling their authenticity.
I'm cheating a bit today and showcasing something I've already done. Manicpixiereadergirl and I decided to do custom Fallout characters for DragonCon this year and had planned on 3D printing our Pip-Boys, rigging them with Arduino's and blah blah blah but ran out of time. Spirit Halloween had a 20% off coupon (which they do just about every week) and I grabbed a pair of theirs. We vinyl cut and applied better decals, filled in a couple of cracks and weathered them. What a massive difference!
So grab anything around your house and weather it. Where has it been? How old is it? Where is it likely to crack, be worn smooth, get chipped? Where would dirt or grime seep into the nooks and crannies? Where is it from? What kind of goop and guck would be dirtying it up? Not sure where to start? Here are some great resources to help you out. Some are free and some have a wee price tag but all are great reading/watching.