A Makeshift Sound Booth you can also Drive to Work
Sometimes you're going to have to use the dreaded ADR (Automated Dialogue Replacement) to fix bad sound in the field. Also known as dubbing or looping, this process involves your talent watching themselves and re-recording their dialogue to match their lip movements. I refer to it as dreaded, because unless you record it in the same spot as originally shot, it will always sound different to the trained ear. Untrained ears won't care.
If you can't record in the same spot, you'll need some kind of sound booth. If you don't have access to one, use your car. It's full of sound abosorbing materials and angles and does a good job on the fly. Just don't record near the freeway, and please don't do it while driving.