It strikes me that recruiting for trials is very similar to the process used to market any product or service. I really recommend engaging a strong marketing partner. It's all about the funnel. First, identifying the target market (who do we want to recruit), then determining how to best reach them. In order to get one person out the end of the funnel (meaning into a clinical study), how many need to be "interested" to even enter the funnel? And where is the best place to reach this target audience? So marketing concepts like reach, awareness, consideration, and "purchase" indicate the move through the funnel.
this might sound a bit abstract, but honestly, a good marketing professional or organization could approach this in a structured manner with success. For example, they would look at how to reach the target - thru doctor's offices? through social media? through education opportunities, etc.