How do those of you who sell your applications on either a subscription basis or with an annual maintenance and support agreement post-licensing handle the fixing/distribution of customer-reported bugs AND the development of enhancement to the product?

Let's assume that you have over 50 installations involved and more buying in all the time.
Further assume that the entire development staff consists of 2 persons and it is this staff that does both bug fixing and feature additions.

Your input appreciated.

Jim