Just nip the growing tip off them when they have about 4 leaves. This will make them more bushy.
I like lots of flowers on mine, however if you want large blooms for showing at a local show then as the flower buds form pinch the side ones off to leave just one to grow large.

Don't waste the extra shoots but cut them off close to the tuber and pot them up as dahlia cuttings (when they are three or four inches long) That way you will get a whole lot of extra plants for free.