A lovely video here by Izzy Hyman, who demonstrates several places and methods for hiding a lav mic on talent. He focuses mostly on male talent placement, but the same principles can be applied with similar clothing circumstances.
A great point that he makes is that if you can, don't hide the mic. The best sound will be from an exposed capsule, so if the client/shoot allows for it, let it show. This if course won't be the case for any narrative work, but for news/docs/etc... there are many times where it can be acceptable to see.
What is a Lavaliere microphone?
Lav mics are an on-body method for capturing sound. You see them all the time in the news clipped to newscaster's ties. They are great because you can get them in really close to the vocals where a boom pole frequently can't get to. They typically are omni-directional, meaning they capture sound all around them, unlike shotgun mics which typically focus on sound directly where they are pointed at. Lav mics can get away with this because the closer a mic is to the sound source, the lower you can dial in the levels, which means extraneous sound will also be lowered.
Most of the time Lav mics are part of a wireless system, because having an actor/talent hard wired to a camera or mixer/recorder is a recipe for damage.