A perfectly reasonalble question, and, in fact, one I'm dealing with right now.
In general, one can either sell the practice and walk away, bring in a younger associate or partner with plans of a buyout, or sell the practice and work for the buyer as an employee. I'm currently considering the third option.
A younger dentist down the hall from me recently expressed his desire to buy my practice and merge it with his. We're friends and I consider him a colleague, not a competitor. It would be good for me because I could continue to work a few more years, but gradually decrease my hours until I'm ready to hang up the drill for good, and it would free me up from all the administrative crap involved in running a practice.
Right now, we're waiting to see if the guy with the office between ours (a psychiatrist) would be willing to relocate to somewhere else in the building so that we could break through and make one big office. This will probably not be a problem, so I'm looking forward to the deal going through.