I would take just some normal peasantry looking garb, stab it with a kitchen knife a few times, then bury it in the ground for a bit. Like more that a week. Flick it out a couple times when you reclaim it, so's no spiders or centipedes, but don't wash it. You want that dirt.
(Edit) You could also try staining it with like olive oil, maybe some bbq sauce or something to give it that added Something's-been-decaying-in-this look.
I have done this for a Halloween costume but with mundane clothing and it worked great.
Not overwhelmingly so, no facepaint required.
And maybe for an event. *shrugs* I may be wrong here, but if you mix oatmeal in with face-latex stuff, it makes a good rotting skin look (I seem to remember this being my dad's trick so I'm not sure).