The black art of shimming

Got the top sill of the rough opening chiseled out. Weird thing: you buy the right tools, and everything goes a whole lot easier. Not that there’s a specialized chiselling-out-the-top-of a-door’s-rough-opening-tool (I wish there were…), but I bought a nice broad chisel, and a 2.5lb sledgehammer, and we’re golden. Mind you, the neighbours are probably pissed at me, with all the late evening hammering.
So the door’s in the space now, with a little more than a half an inch on all sides (in some spots, more than .75 of an inch). It’s a weird situation because the opening’s too small to put in an actual jamb of any kind, and the opening’s not even plumb on one side — the door kind of looks crooked, even when it’s not.
BTW, anyone know how to shim a door? It seems to open and close fine, but when I put the shims into place, the door starts to rub. If I shim too lightly, the shims fall out as soon as I open the door. I’ve got to read up on this a bit, and see if I can get this to work properly. I’m thinking it’s twisting in it’s own frame, or the frame’s normally not square, or something weird.