How to design research participant recruitment flyer to maximise participant response?
Not exactly an answer to your question, but some thoughts on why we don't use flyers.

There is a lot of research going on at our university that involves recruiting participants. From psychology to medicine and economics, everybody vies for the time of the several thousand students. Then there are all those flyers for parties, political initiatives, religious get-togethers etc. etc. On top of this information overload, most students are extremely short of time and mental and emotional energy. Studying here is hard, and students don't want to sit in a lab, when they have the time to not sit over their books.

Our department has the most success at recruiting participants with setting up a small screened area inside the refectory/cafeteria (where the students eat their lunch), standing in front of the entrance, approaching everyone, and testing them then and there. If we cannot test them in the cafeteria, we still approach them there and try to arrange a date for a lab test. We also offer sweets with the recruitment (they're in a small tray and offered when we approach the person).

Maybe that gives you some ideas.