There's a checkbox in the AtB options to allow modifications to Retirement Rolls. That activates a line at the top of the retirement screen which allows you to set a global modifier. For initial contracts, I generally set that to -2, and change that to -1 for the next 2-5 subsequent contracts, before setting it to the default "0", by which time you should have a high enough unit rating and Leadership value to help retain personnel. Then you have the + and - arrows at the extreme right of each member's entry, which allows you to set different odds for the individual member, such as your overall unit commander (I HATE it when the overall commander decides to bug out and abandon his own mercenary unit after the first contract), which I change to a -1 for a couple of the unit founders. Third, you've got check-boxes to allow a month's pay as a -1 reenlistment bonus.
The highest ranking officer's Leadership rating also affects the odds of retirement, and having more personnel than his rating allows will increase the odds of retirement. Keep your personnel numbers down to a reasonable amount.
In my current campaign, I'm usually seeing retirement rolls of 3+ on Green personnel, with significantly worse odds for the Veterans. I tend to hold off promotions for at least one or two Green techs in each category, so I can promote them to Regulars in case I lose too many Regular and Veteran techs, or my lone Elite (I avoid promoting more than one to Elite, because retirement becomes almost inevitable). A mix of Regular and Veteran personnel, with enough Green to fill any critical gaps left by those who retire, seems to be the optimal solution, at least in my case.