I started out looking for something specific, but wound up with a generic thought on shirts. I've had some great work shirts, some really crappy work shirts, so I was looking for what's the big difference between the good work shirts I've had and the nice ones guys get at Macy's. When I see a guy walking down the street, there's a certain feel for a workman's shirt versus a dress shirt; collar and cuffs aside, here's what you've got to work with:

1. Neutral, cool monochromatic vertical stripes. Bright horizontal stripes are for rugby players and high-end auto mechanics; vertical, cool stripes are for a mechanic who works in a garage that both pumps gas and contains a 50-year-old man's nickname in the title. They should be thin and monochromatic, nothing distracting. Note that's 'monochromatic:' alternating with white or black is OK, but not quite right. All blue, all gray, all green, different shades of a single color works best.
2. Two button-down breast pockets. If you even get a pocket in a dress-shirt, it's non-functional unless you keep a pretty booger-rag folded up in there (also non-functional, as I learned the hard way). work shirts have big, utilitarian breast pockets.
3. Square bottom. If you wear it untucked and it has a big flap over the butt, it's not a workshirt. Leave it untucked if the environment calls for it.

Things to avoid:

1. Polyester. I know, it's lighter for summer work, and if your shop buys you the work shirts it might be their only choice -- here, we're buying for fashion, so take some time to find something comfortable. Thread counts are low in work-shirts, and the higher the poly percent, the worse it'll feel. If you look around, 50/50 blends are out there, but 65/45s aren't too bad...when you get above 70/30, in my experience they get shiny and itchy.
2. Country pointy-pockets / mother-of-pearl snaps. The cowboy thing isn't all its cracked up to be, and the style has become such a poser wanna-be look that it's to be avoided at all cost. Trust me, if a genuine working rancher has mother-of-pearl snaps on his shirt, it's his one go-to-town-and-meetin' shirt, and the rest of the stuff in his closet is designed to be beat all to crap.
3. Stuff with other people's names/businesses embroidered on it. I'm sure it's fun to be all "Edgy" and "Ironic" to wear a factory-reject work shirt you found at the consignment store identifying yourself as Bud from Clem's Amoco, but it doesn't make it so. You're as bad as the guys whose shirts say they play for the Aeropostale baseball team. Nobody's impressed.
4. Epaulets or pleated breast-pockets. Not sure why they put them on some work-shirts; they make you look like an off-duty cop. Go smooth, simple, casual.

Since I like to point out frugality: these shirts usually run for less than $20 a pop; show some blue-collar chic and save some scratch, and you can look like you do some work once in a while.