Tips on using wireless microphones

Always start with fresh batteries in the transmitter and receiver.
Most radio mics use 9v alkaline batteries. A new battery reads approximately 9.3 volts, and will gradually wear down to 5 or 6 volts. However, most radio mics begin to function poorly just below 8 volts. So plan on changing batteries every four or five hours, depending on your particular make and model. Experience will dictate how long you can safely go on a single set of batteries.
Good antenna placement is crucial for optimum wireless mic performance.
Give careful thought to the line of travel between the transmitter and the receiver. Avoid transmitting through obvious obstacles such as solid metal objects (inlcuding lighting stands, chairbacks, furniture). Avoid proximity of electronics, including computers, televisions sets, neon signs, and any lights that have ballasts. Human bodies also absorb RF energy, so take that into consideration as well.
The most convenient site to mount the receiver (antenna) may be on the camcorder or sitting on your soundcart, but it might not be the most efficient in terms of signal path!
Camcorders are notorious for blocking the RF signal as well as for emitting electromagnetic interference of their own. If your receivers humm when attached to the camera, try moving them around the camera to find the "clean" spot! Connect your audio with good, balanced, XLR audio cables. Even if your receiver output wire terminates in XLR, some of those output adapters may be unshielded or unbalanced and allow electronic noise to penetrate on the radio mic side of the XLR connector. Try using a short mic cable to go between the radio mic's own XLR and the camera.
A similar problem with humm can occur when you plug a receiver directly into an AC powered mixing panel. A lot of panels emitt a slight magnetic field, so use a short mic cable to achieve at least a foot of distance.
Improve the line of sight between antennas by positioning the receiver either higher up and/or closer to the transmitter. Use a non-metallic pole (such as a broomstick) to hold the receiver 10 to 12 feet high, so it can look down cleanly onto talent rather than having to look through a lot of set obstacles.
Consider placing the receiver closer to the set, or even hiding it within the set. Run a long mic cable back to your camera or soundcart.
Some brands of radio mics allow you to remote the receiver antennas. For instance, the Audio Technica ATW-u100 series utilize common BNC connections. A short length (under 20 feet) of BNC to BNC antenna cable can be used to separate the antennas from the receiver itself. Note that antenna cable (RF cable) is 50 ohm, and is DIFFERENT from BNC to BNC video cable (75 ohm).
When rigging talent, be careful not to allow the mic cable to cross over the antenna wire. The antenna wire on the transmitter should be kept somewhat straight; avoid letting the wire just droop over itself.
It is perfectly okay to invert the transmitter so as to allow the antenna wire to hang straight down. There is no problem with the mic cable looping over itself, or even being balled up with a rubber band to eliminate excess length.
A trick for maintaining the "posture" of the antenna wire is to fasten a thin rubber band to the end of it, and then use a safety pin on the other end of the rubber band to attach it to wardrobe. Leave a little bit of slack. The rubber band will stretch to adjust for body movement; and it should be thin enough to break if extreme tugging should threaten to yank out the antenna wire.
The vast majority of wireless mics sold these days operate in the UHF spectrum, same as UHF television. Although the marketing types talk about their brands offering up to hundreds of operating channels, it is important to note that what they really mean are hundreds of sub-channels within just a few actual television channels.
As more and more local television stations begin broadcasting in HDTV, there will be fewer unused UHF television channels left open for our radio mics. Do not be alarmed if you discover that one to two thirds of your "hundreds of channels" are beset with interference or very short range, since this is common in some cities.
The simplest way to scan for usable "channels" is to turn on your receiver while keeping the transmitter off. Watch the signal strength (or antenna) indicators on the receiver while listening carefully with headphones. If there is no visible activity AND the headphone feed is completely clean, then go ahead and use that channel. If you see or hear anything, then select another channel some numbers away, and check again.
One final note about using wireless mics: try NOT to use them unless absolutely necessary. Because of all the variables involved, there is always a chance of losing part of the dialogue. When feasible, consider running mic cables to talent; save the wireless for shots when cable-free mobility is essential. You don't need radio mics to interview someone seated right in front of you.