WLAN interferers can be grouped into four categories: things with high water content, metal, RF (radio frequency) generators and dense materials. The first group includes fish tanks, water heaters, hot tubs and such.

Metal-related problems can come from obvious things such as filing cabinets, foil-backed insulation, and metal bookcases. But they can also include not-so-obvious things such as tinted windows that use metal-based films and aluminum window screens.

Things that can generate RF strong enough to overpower your wireless LAN include microwave ovens, 2.4 and 5GHz cordless phones (but not DECT or 900MHz models), baby monitors and lots more.

Finally, dense building materials like filled cinder block, brick, rock fireplace walls, adobe or stucco construction will significantly knock your wireless signal strength down.

See the Wireless LAN Performance Improvement NTK for other tips on improving wireless LAN performance.