aside from simple trick with canned antenna (wich is very effective), you have one other very simple way for increasing your router signal quality and coverage 2 times at least.
enter your router configuration page, go to wireless settings and set "N only" for "mode" or "g only". today, almost every mobile device is capable of working in newer "N" mode and every wireless device is able to run in G mode! working a router in b/g/n mixed mode is worthless and is causing a signal degradation.
also, in wireless "advanced settings" set RTS threshold to 750 and (if router firmware is able to do that) set fragmentation threshold to 375.
for me, this method lead to a 8 dB wifi signal boost through a wall.