I did this with a road bike wheel - first by leaving off the lock nut (I was new to hubs!) and secondly by replacing the spacers and lock nuts with thinner ones. In fact, leaving the lock nut off was kind of OK as the dropout held it in place while riding - most of the time.
In the end though I cold-set the frame (see Sheldon Brown) with some threaded rod, a couple of washers and two nuts. You just wind it out to the new width. You can only do this with steel though - not aluminium or carbon fibre. Should you be lucky enough to have a frame made from 753, you can't do it to that either.
Be careful though - overdo it with either approach and you have a broken frame or a locked rear wheel, neither of which is often pleasant.