It is a valuable exercise to get sales people to evaluate themselves on a regular basis and identify how they can improve.  However self evaluation often needs to be taught including how to score themselves and then from that evaluation how to ensure they improve.  If you can instill a culture of improvement in the team they will learn to embrace the process and enjoy the rewards it brings in terms of self development and increased sales success.

When ever anything major happen within the sales process such as a pitch, a presentation or a successful sale the sales person should self assess.  They have to learn to be completely honest and score themselves one to ten.  The next stage is look at how they can improve that score so that when they next rank themselves they see an improvement. 

If the sales team can learn to be completely honest with themselves and rank themselves each time they carry out similar tasks they will soon be able to gauge their improvement.  If there are areas then that they struggle to improve upon you can discuss these at reviews and perhaps look at executive coaching to help the sales person learn the technique they need in this area.