Listening devices and wireless microphones become progressively smaller with advances in technology, and thus more difficult to detect. They can be hidden nearly anywhere with little chance of being revealed, although some places are better than others for effectiveness in picking up sound. If you're planning to hide microphones, be aware that this practice is illegal in many situations and get you into a lot of trouble.
Hiding a microphone in a public location is simplest, because you have easy access to such places. Do it when nobody else is around to avoid the possibilities of being seen and having the device stolen. The main challenge is to put the microphone in a location where it will detect useful sound, such as conversation, while not being overwhelmed by ambient noise, such as traffic or crowd movement. Place the microphone as far as possible from sources of loud noise and, if possible, inside an area that will muffle irrelevant sound, such as a telephone booth or foyer.
Hiding microphones in private locations will most likely get you into legal trouble, unless you do it in your own home. The number of hiding places for a small wireless listening device is nearly infinite in most private abodes. Find a place that is not likely to be disturbed by anyone, but that isn't so far from activity that it won't pick up sound. One prime location is in the upper shelves of a bookcase. A shelf full of books presents a lot of visual information, so that even if the microphone is slightly visible it isn't likely to be noticed by anyone who isn't looking for it.
On the Body
The biggest challenge in hiding a microphone on the body is keeping it hidden while not placing it under clothing that will create noise that destroys sound quality when you move. Some spy microphones are created to look like buttons or other parts of clothing, so they can be worn on the outside of the clothing without arousing suspicion. With the advent of ubiquitous personal electronics, such as cell phones and iPods, it is now very easy to incorporate a spy microphone into something like this and keep it in plain sight without arousing any suspicion at all.
Inside Other Devices
Listening devices can be quite easily hidden inside other devices. Because most electronic devices are built with a hard case surrounding a cavity that has space, as well as electronics, a listening device is easily incorporated inside the hard shell of a device, where it will be effective but undetectable without disassembling the larger device. Desktop and laptop computers, televisions, stereo systems, cell phones and radios are all effective devices in which to hide a microphone. One problem is that if the device is activated to play sound while you are attempting to listen in, you're not likely to hear anything but what's playing on the larger device.