Working on retainer

Can anyone offer advice about working on retainer? Stuff like:

  • The ins and outs of how to do it
  • How do you invoice?
  • What happens if the workload for the month is less than expected?
  • What happens if it is more?
  • How you approach a client with this option?
  • How is this contractually worked out?
  • Any success/horror stories?
    Any advice is appreciated… Thanks.