Regardless if you are in a firm or in an internal recruiting role, trust is the brand you are selling. I have walked into offices of new clients, as well as internal hiring leaders new to work with me who had a bitter taste of past recruiters. 'Tis true...there are bad recruiters who give our industry a bad name. The complaint is often about either promises broken, poor quality of candidates presented, or the favorite act of "Disappearing Recruiter" (where did they go, hello is anyone home, please respond to my emails). No wonder they were bitter when I walked in the door...selling smiles was not going to work on this crowd. Building trust requires a core philosophy, like a secret code you live by. You must be passionate about protecting it, otherwise your reputation will falter.
Establish Credibility: You will have to earn credibility along the way, but you can set some stakes in the ground to let them know successes under your belt. I typically give a quick summary of my background and talk about similar searches I have filled in the past. Setting myself up as a subject matter expert in my field bears some comfort with my new ally.
Set Expectations: Make promises you CAN keep; such as your typical process, how you work. Sometimes this includes telling them a few tricks under your sleeve as to how you search for talent (this often creates a look of awe from said hiring manager - shabam...you just captured their attention). Your job is not to bring in quantity, it's about quality, 3-5 candidates is enough to present (depending on the role). You MUST be sure to talk about quality, timing, and salary target - three things that are important to the search, but your hiring manager can realistically only have two. What is most important? i.e. If top quality and timing is the priority, then you will want to make sure you have wiggle room on salary to be competitive. This will guide your conversation.
Walk the Talk: Whatever you just committed to, now you have to earn the trust by following through with delivery and process.
Communicate, Communicate, Communicate: Even if it's a quick email update or a voice mail to say where you are or where you are struggling, make them a partner in your search. If you need to alter a posting, alter the salary, post somewhere new to gain traction, COMMUNICATE. Talk to your hiring manager, truly make them a partner in the search, this is their hire, they care about what is happening.
Establishing trust with your leaders will make them a fan of your brand and will make future searches much easier...not to mention that they often spread the word.