My goal in all of my client engagements is to provide extraordinary value, maintain transparency and keep it simple. As such, I currently offer two models:
Retainer: This is my preferred approach, mostly b/c it allows a client to use me as a general operational resource and seek help as issues arise. As such, it provides flexibility and a more economical approach than a project-based hourly rate. I offer three models based on an estimated, set number of hours per week - 8 hours, 12 and 16 (so 1 day, 1.5 and 2 days, respectively). We use a chosen model as a baseline and track my time to see how much I'm spending, then review quarterly with the client to ensure hours worked approximate the retained model.
Project based is basically on an hourly basis. I expect that model will be used when there is a specific need for my help on a short term basis, likely with a heavier lean on my legal background. Examples might include help with drafting/negotiation of key contracts, financial modeling or complex transaction support.