Replying to Best Methods for Clean Audio in an Interview?

Hello M&A Friends, I need some help.

So I am going out to shoot an interview tomorrow for a documentary I am working on, for the second time. The first time around, my lavaliere battery died before I could start shooting, so I had to use a Rode shotgun. It didn't go so well. I ended up with a fuck ton of background noise, and the master level with music ended up around -12 db, and my goal was -3 to -6. So my question is:

How can I minimize background hiss/hum/ambience while using a lavaliere clipped to a shirt? I have heard that auto gain control makes the hiss bad, but that turning it off can screw up the audio by clipping it all over the place because it doesn't adapt to varying levels of speech. I want as little background noise as possible without clipping her voice or making it too quiet. I will be able to reduce it some with audition, but I want to shoot it right.