EXPERT RESPONSE
The 2.4 GHz ISM band is a very crowded portion of the RF spectrum. In addition to microwave ovens and Bluetooth devices, your WLAN may encounter RF interference from DECT or TDD cordless phones, wireless video cameras, and of course other Wi-Fi networks. Of these potential interference sources, TDD phones have the most drastic impact on Wi-Fi performance. Wireless video cameras and microwave ovens can also cut Wi-Fi throughput in half, although they must be located fairly close (within 20 feet) of the affected WLAN devices. DECT phones and Bluetooth devices use frequency hopping and so have less of a continuous impact on your WLAN. To learn more, read this Farpoint Group study on the effects of interference on general WLAN traffic.
Where do other Wi-Fi networks fall? Interference from other APs tuned to the same channel (or an adjacent channel) can cause just as much performance degradation as a nearby microwave oven or video camera. Worse, they can degrade throughput at greater distances, and may be much more common in dense, urban areas. Because it's easy to spot Wi-Fi co-channel interference, you might as well start by looking for other APs on the same channel and then change your AP to a less congested channel. Try to find an unused non-overlapping channel, because each 2.4 GHz channel frequency is actually the center frequency of a channel that's 22 MHz wide. That means that an AP tuned to Channel 6 causes interference on Channels 4 through 8. In the 2.4 GHz band, Channels 1, 6, and 11 are the only non-overlapping channels. If none of those channels are unused, choose the channel with the lowest co-channel interference -- that is, the one with competing APs that have the weakest signal.
Finally, interference is not the only reason for poor performance. Clients may be too far from your AP; obstructions between the client and AP may be reducing signal strength; or nearby objects may be reflecting the signal enough to cause multipath degradation. If you suspect these problems, try changing the client's location to reduce distance and obstructions, or increasing the AP's transmit power. With these kinds of problems, you will clearly see low or fluctuating received signal strength at the client, as opposed to interference which degrades throughput even when signal looks strong.
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