Standards said:Perhaps only offering people a limited signing on extension, ie person gets promoted to Cpl after (lets say laughingly) 6 years, If they are signed on for 9 at the time, offer them 3 extra years of service. This could mean that if they did want to carry on up the ranks they would have to continue to prove their worth. If they did not they would know that their service ended at the 12 year point. Carry this on with the next rank, get to Sgt before 12 years, offer another extension and so on.
Basically how it worked when I joined, you could only join initially for a max of 6 years (9 for techies) then had to be promoted or recommended by your hierachy to sign on further. Prior to that we had 22 year SAC's, they had no interest in promotion, most had 2nd jobs and they were happy! The problems came under various "Options for Change" in the late 80's early 90's where people with 8's and spec recs were being signed on to 15 years to enable them to get to Cpl! You have a different problem to other trades due to the nature of your job and (too) young SNCO/WO's do cause a bottleneck, you have my sympathy (not too much though, can't let the side down now can I?)