WiFi
Selecting a Channel
When selecting the channel for a 2.4 GHz WiFi network, you want to choose one that is not used by another network or the least number of other networks possible. However, you should not just choose any channel: There is an overlap between channels and it is better to choose the same channel that is used by another network than a different channels that overlaps with a used channel. There is an excellent explanation of this problem at http://www.metageek.net/support/why-channels-1-6-and-11/. In summary, you should usually choose channels 1, 6 or 11 because they do not overlap.
For 5 GHz WiFi, the same considerations apply. However, due to the fact that 5 GHz is used less often and has a shorter range, you will usually experience less interference at this frequency anyway.
Things get even harder when you use 40 MHz wide channels (refer to http://www.connect802.com/80211n_channels.htm for more details). In general, using the 40 MHz mode in the 2.4 GHz range is not a good idea because of interoperability issues with legacay devices.