you may want to consider prosecuting DUIs or defending them as a PD for a while and then move to a firm.That can be a very lucrative type of practice from what I've seen.
Nearly all criminal law "firms" are solo practitioners or small partnerships. Few hire clerks or associates. Your best bet is to get in with the DA first. Practice there for a few years, and then after you have some experience, open up your own gig or partner up with somebody.
Any other info would be greatly appreciated.