Pruning your wisteria will prevent the size and shape from growing out of control, but it will also help the flowering process. Due to the rate that the wisteria grows, you will need to prune it twice a year, once in the summer and again in winter once the leaves have fallen.
Once flowering has finished (they tend to flower between April and June 3-5 years after planting) try to tie up any new branches to support and guide them into the shape you would like. Once you are happy with the shape, cut back the green shoots of that seasons growth, about 3-5 buds from the base, this helps keep the size under control and helps the flower buds grow rather than just greenery.
Once pruned you may find a few of the branches die, this is normal, you can either remove these entirely or cut them right back.
Pruning your wisteria in winter will encourage regrowth in the spring time, the best time to do this is just after the leaves have fallen in November. You should start by removing any dead or diseased branches. Once they have been removed you can cut back the longer shoots that have grown in summer, within 2-3 buds, tying up any that need to be along the way. Cut back any of the main branches if needed to the desired size.