If you haven't, find out why they are not paying and try and work it out, saying something more than "pay me" can be surprisingly effective, do this through email/mail so you have a written record. If that fails, have a lawyer send them a letter. If that fails than small claims court. The first two steps will put you in a better position for small claims court and make it less likely it ends up being your word against theirs, get that documentation.