This fantastic video from Izzy Hyman is great not just for filmmaking, but for any time you need to mic somebody up and the lav can't be in the shot. I'm not a huge fan of lavs, even less so when they have to be hidden, but if you shoot video, or you're getting into audio, at some point you're going to have to do some of these techniques. The triangle tape trick has been one of the most useful to me, and if you've never seen it or used it before, you should absolutely try it the next time you need to hide a lav. You can also use little adhesive discs or mic concealers that are made specifically for this purpose like these here and here, though there are plenty of times where some regular old tape can do the trick just fine.
If you've got more techniques not mentioned in the video, be sure to share them in the comments!
Source: Izzy Hyman