I don't have a comprehensive plan to share, but a few things to consider...
1) What kind of mentors are you looking for? Even if the answer is "anything and everything" make an effort to identify specific skills that are critical to your success and that you are currently lacking. Once you've identified say, two or three, key critical skills that you need you can better target where you might find such an individual with those skills that may be interested in mentoring. You can also get the kids to think of people in their network (parents/neighbors/family friends/parents of their friends/friends of their parents/etc.) that have those skills, as a personal appeal from a student may be more successful than recruiting unaffiliated adults.
2) What are the expectations of mentors on your team? When are meetings and how often are they? How often are mentors expected to attend meetings? How many students should they expect to be working with? Which other mentors will they be working with/reporting to (if you have a hierarchy)? You will have better luck attracting and retaining adults if the expectations are clear. You don't want them to feel as if they are jumping into chaos land (nevermind I'm pretty sure that's what all FRC teams actually are). Clear expectations are the start of a happy relationship
Be prepared to answer such questions, and if they don't ask be prepared to supply such information.
3) Consider the process your team has for introducing new mentors. My team's process is three-ish steps. A potential new mentor first meets with our key adult mentors where we discuss the answers to questions I listed in point 2, and where we ask what the mentor would like from their mentoring experience. The next step generally involves a tour of our space, an overview of how we operate, and the opportunity to observe a meeting and meet the students. Finally, we devise a project (targeting the skills that we identified in step 1) and assign a small group of students to work with our potential new mentor. The project is something that's valuable to the team and appeals to the mentor (hence why we asked what they were hoping to get from their experience). Also, SMALL group of students is key. You are less likely to hold the interest of your new mentor if they are overwhelmed with too many students. The lead engineer also plays a part in the initial project as the adults learning how to work together is just as important as the adults learning how to work with the students.
4) If you look and act professional you are probably more likely to attract and retain professionals. Is your build space a disaster zone? What are the behavior expectations of the students, and are they enforced? How do the students interact with new adults that attend meetings (do they ignore them and do their own thing, or do they eagerly try to show off what they are doing)? Do you have clear goals for your team performance (so potential new mentors can see how they fit into a vision)? Do you have calendars and schedules published (key for adults that are mentoring around work schedules and need to take time off for events)?
~Allison