There is potential of radio interference when working with wireless microphones, so be sure to get a unit that offers the ability to use different frequencies. Most kits include a lavaliere microphone, an XLR adapter for other microphones, and a wireless receiver to plug into the camera.
You need to be aware of a recent development regarding the use of wireless RF microphones. As of June 12, 2010, the Federal Communications Commission (FCC) has made it illegal to use any equipment that operates in the 700 MHz band. This set of frequencies has been reassigned for use by emergency personnel only. Many wireless mics previously on the market operated in this frequency range and must be replaced. More information about this ruling can be found on the FCC website at www.fcc.gov/cgb/wirelessmicrophones.
Be certain to actively monitor your audio with headphones. Besides interference, there are a few common problems. One is that batteries can wear out, which can introduce dropouts and noise. The most common problem, though, is human error. With two off switches (one on the microphone and one on the receiver), it’s easy to leave the microphone turned off. Remember, you plug into the camera and listen to what the camera is recording to know you are getting good audio.