I am a marketing consultant based in San Francisco with more inbound requests than I can handle. I am well aware that I could make this an agency, but I am not sure that my margins would support the work. How have other solopreneurs scaled their consulting practices?
Sounds like you've reached a crossroads - with more business than you can handle you have what many would consider to be a "high class problem".
The key to scaling is to build it in to your business model... With that said:
Might I suggest you start with re-visiting (or, if you've never formally done so - creating) your business model.
Since margins are an issue - you will have to consider restructuring if you are going to be able to scale. How elastic is your pricing structure? Can you viably charge more for your services? Can you offer additional services or products to create more margin via an ascension model or via back-end sales?
Many of the entrepreneurs I work with jumped head first into their start-up without considering what would happen during stages of growth. Those that are successful enough to survive (and thrive) often reach a point where they have to "go back to the beginning" and re-vision their business if they are to mature into a successful business (where you, the owner, build an asset and get compensated by the business for creating it via distributions).
You know how to reach me if you'd like one-on-one help.
In any case...best of luck!!