On the farm, I wear a ribbed cotton tank top, one that looks good, but is washable and easily replaceable and comes in every color imagineable. If it's really sunny I wear a light cotton button down over it so I don't get fried.
For pants I used to have those kind that zip off at the knee, but I can't find them anymore. Wrangler jeans have styles that don't make you look like an icecream cone now, and they wear really well. I buy carpenter jeans, too, for the pockets. I hear that Dickies work pants also work pretty well.
My boots are these, kind of a cross between hiking boots and work boots, they don't have a lot of tread on the bottom for mud to get stuck in, but they have enough so I'm not sliding all over the place. They're really comfortable, and you can get a waterproof version.
I wear a straw cowboy hat to keep the sun off my face.
I have the worst luck in gloves, though. I found some that I adore, that actually fit, with leather palms and fingers but a breatheable back, and I've worn through them, but now I can't find them anymore.