IMHO, a bonus, and a real bonus is probably the way to go.
For this size of operation, say $20K for F/O's & $30K for Captains. Make it payable every 12 months, and completely dependant on the full years service. Leave even 1 day prior to the 12 month period and you don't get a zack!
Forget about the strings (3 months notice, what nonsence), people see through that kind of cynisism and will reject it, especially in todays dynamic employment environment.
Those who stay to take advantage of the bonus will more than likely plan their future financial goals around it. You will probably "hook" them for years, if not for life.
Those that move on regardless, and good luck to them, won't cost the company a cent!
If we're talking about the viability of the company, a real no-brainer I would think.