There is an old saying that is very appropriate -- "The best time to fire someone is before you hire them". That speaks to the importance of having meaningful interviews with candidates, not just seat-of-the pants discussions. We need to ask probing questions and stop accepting fluff answers like "I'm a people person and I want to make money". What would you expect them to say - "I hate people and I want to be destitute". Have a well thought-out list of open-ended questions and you can often uncover the real person before it is too late.
Always be hiring. Have you noticed that the people who are good at conducting an interview are the people who do it all the time. I agree with David Martin, you need to have a plan, a list of good questions to ask and layer from. Don't accept fluff answers. But it takes practice to get good at actively listening to spot the person just giving the answer they think you want to hear. Last note - if you could replace your worst employee with one you knew had the skills and personality of your best employee, would you do it? Always be hiring!
Hiring: You should be constantly advertising, recruiting and interviewing applicants. Since most hires in the automobile business are done because of need and time is of the essence, we hire the wrong people for the wrong reasons. Since everyones expectations are low going in, pay plans for new salespeople are extremely risky for anyone to leave a job for the unknown. If the business is not confident that the employees will succeed, how can the employees be confident. Majority of hires are unemployed people who need a job and are willing to take the risk, because there is none. Hire for quality, not quantity. Screen the applicants for sales ability and never stop training them. Set minimum standards for SSI, volume and gross and put them back into training before letting them go back on the floor, if they fall below these minimums. Two months under the minimums and the employee is terminated. Inspect what is expected and don't allow your top performers to pick up their slack and allow them to exist.
Firing: See above procedure!
Don't be afraid to make a mistake. This remains a people business and therefore unpredictable. And, yes, while there are a multitude of predictive indexes that can be used to help in the hiring process, even the best are not bullet proof.