I produce short documentary videos, and often need to film interviews outdoors. I typically use either a lavalier or shotgun mic. My lavalier is good at isolating the subject's voice, but is vulnerable to wind. The shotgun—once I cover it in foam and a "dead kitty"—doesn't have wind interference, but does pick up ambient noise. I'd like to minimize both the wind noise and ambient noise. How might I do this?
There are a few ways to do this.
For one you can pull it out in post production with some mixing tricks. EQ and noise gates can be your friend here. Keep in mind that you wont want to go too crazy since some background noise is nice.
You can try tucking the lav behind a collar or in the neck of the shirt. this may help to block some of the wind. You can try one of these, looks like it may work but I have never used one so I don't know.
You can wait the weather out. This of course is not optimal but you can wait for a windless day to film (if you have that kind of time).
You can play with the shot so that the subject has their back to the wind. This may break the wind over them and help mitigate the noise at the element.
Reducing the ambient noise on the shotgun mic can be tough as you have less control over that. Again it can be touched up in post but I can't think of any ways to seriously mitigate it in production shy of actually having control over the noise sources. You may find that boosting the voice (using the lav track) is enough to make the ambient noise sit well in the background. You will want to keep some of that ambient noise as you don't want the soundscape to seem plain.
The whole idea of the shotgun is that it mainly picks up ambient noise that is in the direction it's pointed. Have the speaker stand with their backs to the quietest area around. If there are trees, walls, hedges, etc., that's where you want the speaker. A truck rolling by or a jet flying over is going to get all over your audio no matter what, so you have to wait that out. Road noise in general is usually a big problem and your best bet is to get as far away from it and have the speaker face the road. Sometimes it helps to bring the boom below the sight line of the camera and point it up at the speaker's mouth, so that the background behind the speaker is the sky, which won't reflect anything.
Lavs are tricky things and a lot of times when two people are talking face-to-face, person A's lav actually picks up person B better than person A (and vice-versa). This means if one person is talking, the lav will pick up things in front of the speaker better than the speaker. In this case you want the speaker facing away from noises, which is counter to the normal positioning for a boom (with shotgun).
The shotgun is almost always the better quality audio, especially if you have a competent boom operator. The lav would normally be a safety for the boom sound, and might be mixed in to fill out the low end of the voices since the boom can be a bit lacking in that area.
The more you want high-quality audio, the more you want a great boom operator, and the more you want to listen to them. When they say they didn't get the sound or there's ambient, you want pause until they are happy, and take their advice on where to film and record. The challenge and also beauty of documentaries is that there's no ADR that can be done later, so it's important to make sure you're getting what you need right at the start. A boom operator's entire job is making sure that happens.