For a general yellowed look, you can use the tea bag method mentioned previously... though I like to use coffee grounds (e'en if the smell is stronger... it's less likely to wash out after the first few times).
For makin' the garb DIRTY (as in muck and mud and stuff like that)... at work we use this pre-bought stuff called "schmutz" (though I dunno if that be a nickname or the product's actual label), put a few handfuls in a torn-off piece of wifebeater shirt, tie it into a bundle and use it like a pommel. Be sure to really grind the stuff in, and get the spots that are the most likely to be dirtied... that means the creases and seams, especially. for pants, be sure to get the waistband, the crotch, and the hems of the pant legs. for shirts it's sideseams, collar, "the pits," and the hems of the sleeves. If yer cheap like me, ye'll just use real dirt. Ye get that fine earthy smell! But the actors don't want to wear real dirt, so we have to use fake stuff.
Spoiled gits.
If ye want to go all out, you can use rubbed in baby oil for grease. A word o' warnin' it feels nasty an' doesn't last very long (once the oil dries, the stain disappears, so if you grease the clothes the night before a faire, keep it in a plastic bag until the event). Be sure to wear some manner of plastic or latex gloves though, as baby oil stinks like bloody hell if on your cuticles for too long!!
For all agin', just use common sense and get the spots that you would think wear the most. Places that wear easily (the seat of pants, the hems of pants, the collars of shirts.. that sorta thing). Only you can know where your rubbin' and slidin' all the time!
Mind, all of this is for a quick, one-day aging process. If you want the real thing, do as the others suggested and just live in the garb! But if ye want an overnight method... here ye have it, the protected secrets of the Sony Studios Wardrobe Dept!! (Just don't tell on me, they'd keel haul me if they knew I were spillin' their secrets!)
Have fun!
--Mab