for maintenance or incremental work i'd offer a retainer.
i did that with one client. i started hourly and eventually once we had a good working relationship suggested to switch to a retainer. i actually let the client pick the fee. he picked the same amount that he had paid in hourly rates before. i figured that if there was more work then i'd renegotiate. but what happened was that my work went down, yet the retainer stayed.
that's the big advantage of a retainer.
with hourly you get more if you work more and less if the work is reduced.
with a retainer you don't work more than the limit of the retainer allows, but the less you work, the more you make for the time worked.