[thijsc]

In addition to charging a retainer it would be wise to not take on fixed scope/price projects.

We always tell our clients that we charge by the hour and we give them as accurate an estimate as possible. Usually there's a provision that we warn them in advance if we're going over an estimate by more than 20%.

On occasion I've found it helps to take on part of the project for a fixed price as a first trial run of your services. But then always be clear that it's a trial run and that you'll automatically fall back to the hourly rate for any work after that.

We bill by the month, regardless of if projects are finished. This helps a lot too since the client can't use the date of the invoice as leverage in a negotiation about scope for example.

This strategy works out great for us, I can enthusiastically recommend it.