If your sales team is made up of good salespeople they not only pay for themselves, but they contribute to the profits of the company. If they aren't, then you should have terminated them for performance and maybe you wouldn't be reducing your workforce now.
The only case when a sales force may need to be reduced is when two companies merge or one acquires the other and sales territories overlap causing a duplication of duties. That is really the only situation to take a chance of losing a good salesperson.
In the event you find yourself in this situation it's important to convey to the salesperson that his termination doesn't have anything to do with his performance. Explain the situation and be as open as possible. Take time to answer questions and understand his position. He's done his job — sometimes exceedingly well — only to be without a job now. It's a very frustrating position to be in.
If your company allows, offer to write a letter of recommendation spelling out the reason for the reduction in force and note that the salesperson would be eligible for rehire (although he may be hesitant to give you another chance). The point is that you handle this termination differently from one where the employee brought it upon himself based on his actions, behaviors, performance, or lack of performance.
Reduction in force, downsizing, right sizing, layoffs — call it what you want, this can't be stressed enough how you should make every effort possible for it not to impact a salesperson who's productive.
When you've made the decision to let someone go (for whatever reason), write out what you want to say just as you would a sales script. Just like a sales script, this only a tool. Never sit down and read a termination letter to anyone. Give him the courtesy of speaking to him face-to-face, person-to-person and try to not make it some cold, sterile action.
The main purpose of the face-to-face meeting is to make it clear the other person knows he's getting fired. Someone once had a coworker sit down to terminate someone and in her desire to not hurt his feelings and soft-pedal everything, the two talked for over an hour and the employee still didn't know what was really happening.
They talked about the employee's lack of performance, how this should've happened, that should've happened, and so forth. Everything was said except "I'm terminating your employment." In fact, at one time the employee asked, "Does this mean you're firing me?" Keep in mind, this was more than an hour into the conversation! By scripting or outlining what you want to convey, you'll do a much better job in getting your point across.