802.11 uses unlicensed radio frequencies that are freely available for anyone to use, so there is no way to avoid interference. However, 802.11 also uses CSMA/CA, which means that when a radio is transmitting, other radios should not be, which helps minimize interference by sharing the frequencies.
Some things you can do are to only use channels 1,6, and 11 in the 2.4 GHz band, and only use 20 MHz channel widths in this band. However, you should try to avoid using the 2.4 GHz band if possible, as there are only 3 non-overlapping channels, and more devices per channel, so more interference. You can also try talking to your neighbors and ask them to switch to 20 MHz channel widths. It's to everyone's advantage. Less interference and more cooperation = more throughput for everyone.
Look at your client connections to see if they are connecting using 2.4 GHz or 5 GHz. Depending upon where your network is installed (what country), you likely have at least 8 channels available in the 5 GHz band, and possibly many more. Because 5 GHz has more channels, it is less congested and has less interference.
If you want to understand 802.11 in more depth and how it works, look into attending a Certified Wireles Network Administrator class or reading one of the CWNA books (Sybex CWNA Study Guide or Certitrek CWNA Study Guide).
I hope this helps,