I've done this trick many times, and it never fails to illicit some kind of response.
I present it in a comic-noir style (dark comedy), where I'm grossing people out and getting a good laugh from it myself. It always brings out reactions of both disgust where they can't look away, and laughter where they know its impossible and gross.
I prefer this presentation because I don't think David Blaine's patter ("watch... if I just... "[proceeds to pull string from stomach]) really suits my style. There's a plethera of jokes and gags that can be done with this.
I've actually taken an idea from SAW (SEAN FIELDS) where I swallow a ring and some thread, and the ring is on the thread when I pull it out of my stomach (I don't use the word "belly" because it seems to make me think I'm fat :). I haven't performed this yet, but its in the works; if anyone can help me with my idea, PM me and we'll see if we can get this going.
I highly recommend getting Swami Mantra, as there are a lot of good tricks in there of different types, however some of the yoga-maya ones, I wouldn't touch with a ten-foot pole.
Best of Luck
Professional Coin Magician
Studio Director, Alberta Printmakers
Sociopolitical Fine Art Printmaker