I have just set up something for a client that works as follows:

they pay a fixed amount each month for a fixed number of hours at a fixed (lower than usual) hourly rate.  We estimate any new request in terms of hours and we have a target (not guaranteed) turn around time for fixes of five working days.  The hours they pay for are a bank, if they go a bit overdrawn one month they can eat into the followoing month's allowance, if they don't use all the hours they roll over. 

We are at very early days so I can't say if it works but, like all "my" good ideas, I pinched it off someone else and they have been using it for a few years with a range of clients.

feel free to email direct if you want to discuss it further.

A support option or retainer is our incentive to maintain in-house knowledge about the solution.  If a staff member moves on, we have made sure we have invested the time to retain the knowledge.  The idea of a block of regular hours which can be used up over time works but needs active management to make sure they are actually being used up and the client is getting the added value.  It depends on the pace of new feature development.