We used to offer retainers that were a flat cost for a certain number of hours, billed in advance, with no differentiation between strategy and execution. Ex: $3000/mo for 15 hours of work.
Clients could roll over hours from one period to the other so we were continually keeping track of how many hours they had used and figuring out what to do when we went over or under. This was bulky and we replaced it with the .Client Relationship Structure