I totally agree with Nostrum. Gleaming White is only appropriate if the character only ever puts on the armor for parades, or like, it's his first day on the job. If you look at the 501st (the guys that dress up as stormtroopers), nearly all of them do significant weathering on their costume; and their weathering even varies based on what planet they are stationed on.
All sorts of techniques are used for weathering. for the jumpsuit, if using paint, you'd probably do most of it with drybrushing techniques: load up a coarse brush with paint, then paint it along a piece of scrap paper until there's barely any paint left on the brush, and then just drag and dot the brush around on the suit, focusing extra on the places that are likely to get dirtiest; like at the borders of armor pieces. Another possibility would be to just toss it about in a large hamper with a couple hunks of natural lump hardwood charcoal.
For the armor, it partially depends on how durable you make yours. Once you've done all your finishing work, clearcoats, buffing, you have the armor looking all perfect, and you've given the paintjob a couple days to reach maximum adhesion, you start by beating the crap out of it: whacking it with chains, and kitchen knifes (safety glasses and proper precautions of course), tossing it down flights of concrete steps, wearing it while playing contact sports in a dirt lot, that sort of thing. Then you either just rub dirt and grime into it, or for something more permanent, you use washes of acrylic paint mixed with water. You wipe it off of the high spots while it's still wet, so it just settles into all those lovely scratches and whatnot that you've made. You can get mud spatter effects by loading up a brush with paint, holding a stick close to the armor, and wacking the stick with the side of the brush. To get scuff marks, take a hunk of black rubber, like from a tire, and gash away. A lot of it is up to creativity. Have fun with it.