I have added a high gain WiFi antenna but my WiFi datarate has not improved?
(i) Multipath fading
(ii) FCC limits on ISM band radiated power
(iii) Omnidirectional vs Directional high gain antennas
What we have found to be the best (in terms of price/performance) is to daisy-chain routers. The beauty of this is that we can not only setup the second router to operate in a non-overlapping frequency spectrum (just a reminder there are 3 non-overlapping spectrums in the U.S channel 1, 6 and 11) but we can also isolate collision domains.
Just a quick reminder: WiFi media access method is CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance what this means is that if mobile station detects that the medium/air-interface is in use it defers transmission to avoid a collision. This deference is manifested as a drop in datarate. By having multiple non-overlapping frequency channels the possible of collision hence the probability of deference is reduced. The other advantage has to do with the fact that we actually create a second network and the second router actual "routes" network traffic independent of the first (oversimplified description

See High Gain WiFi Antennas and WiFi Repeaters/ Amplifiers