Typically, a sales team is comprised of a small but elite group of stars (10-15%), a majority of middle or "core" performers (70-80%), and a group whose performance trails (10-15%). The middle group often receives the least attention even though just a small gain in this group can generate a significantly higher increase in sales than the same gain among top performers. Research has indicated that middle performers respond to some incentive strategies more than others. For example, multi-tiered targets, acting as stepping stones, encouraged them to strive for increased sales. Others have found success in moving the middle with contests that reward achievements in efforts, such as the most prospecting calls, the most sales visits, positive customer satisfaction surveys, and the retention of existing accounts.
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https://hbr.org/2012/07/motivating-salespeople-what-really-works