One of the hits I got in the past week was "How to carry a gun in a suit."
Simple. Same way you carry in any other clothing. Actually, a suit can often be easier to carry in, since it has a built-in cover garment. Suit coats work wonders for concealing shoulder holsters, as well as OWB and IWB rigs.
I don't wear a suit very often, but I've had my very best luck wearing a tuckable IWB rig, if I was concerned about taking off my jacket. Which, interestingly, is the same result I get in standard street clothes.
And that's how you carry in a suit.