Originally Posted by johnwill

Probably nothing wireless! You are bumping into the limitations of wireless technologh.

I'll tell you what I did for a client of mine.

Use Xirrus Wi-Fi Inspector to find the spot with the the weakest interferring channel first.

Next, configure the DIR-615 to that channel, and disable 802.11n capability on the wireless page to minimize it's channel footprint.

Finally, install something like this Hawking [HSB2] Hi-Gain WiFi Signal Booster, and this Hawking [HAI7SIP] Hi-Gain 7dBi Omni-Directional Antenna on one of the antennas of the router.

The purpose of the boost in signal is to simply make your signal "taller" then the rest of the networks.