For reference, I'm carrying a 1911 in a cheap IWB holster.
As far as cover garments go, just about anything will work. When I started carrying, I gave a lot of thought to it and made it a point to look for long buttondown shirts I could wear untucked and open; Hawaiian shirts, camp shirts, general Eddie Bauer fare, and the like. In my case, this wasn't much of a change from how I normally dressed when given the choice (I like the extra pockets). In cooler weather, I'd wear a vest or light fleece jacket.
Since then, I've realized that I don't need to be nearly as concerned as I was. With the same gun and holster, I wear a loose T-shirt over an undershirt, with the top shirt untucked; a polo shirt, untucked; pretty much anything an inch or two larger than your unarmed size, left untucked, will conceal just fine. Wearing polos and such has the additional advantage of looking respectable enough that nobody thinks I'm trying to hide anything, and a good knit polo will be stiff enough to minimize printing.
Occasionally, I need to dress a little nicer than that; in those cases, usually some sort of a long-sleeve buttondown, preferably with a pattern, tucked in over the gun and bloused consistently around my waist (being a larger fellow, blousing looks better anyway). Works great; sometimes, even I can't tell if I'm carrying.
Bottom line: don't worry too much about it. Just make sure it A) hangs a bit below your beltline, and B) is loose enough to avoid clinging, and you ought to be fine.
but I'm far from worried about that.
