Honestly, many of the wireless frequencies are already gone.
Our church has 10 channels of wireless for lavaliers, and many of the frequency bands are already being used by the digital TV stations for broadcasting.
Most of the time, it's not that big of a deal, but on occasion it's an issue.
Most of the manufacturers will provide you a list of possible interference in your area.
Some tips we have found helpful:
1) keep a direct line of sight between the body packs and the receiver units.
2) keep the distance between the body pack and the receiver to a minimum.
3) FRESH BATTERIES! We switched to NiMH 9 volts so we could keep the packs at a nice strong 9.2 v (yes.. they actually reach 9v) at the beginning of a service, and then charge them over the course of a week.