A good, directional antenna at both ends (facing each other). I have a pair of Hawking Corner Antennas that work Great. I share my internet with a neighbor about a mile down the hill from me and it works perfectly.
These are small, about 8" high and come with a bottom bracket to mount them. They say they are indoor antennas but mine have been out in the weather for a few years now with no problems. They have about 10' (3M) of coax with a reverse SMA connector that fits all routers with the "small" (standard) antenna connector, and they throw in 2 adapters to fit the routers with the larger connector (like many Linksys routers).
You can mail-order them from Fry's in the U.S.A. or Google the name and numbers to find a source closer to you.