We have two clients where we use a flat monthly fee sort of deal.  Obviously this requires a high level of trust and to be honest - in the long run - we lose money.  However, we don't really begrudge the "discount" that the clients take because the stability provided by knowing that we will be paid a designated amount on a designated date is really helpful.  Both 'deals' are fairly complex and flexible, and on one of them we needed to have a contract lawyer look over the agreement before signing.

It's not perfect by a long shot, but in this business it sure is nice to have even a small amount of financial stability.