There is very little that can be done to boost the power on a consumer wireless router. If the router is at one end of the home, there are antenna add-ons that can be created that will increase the signal strength in one direction. Freeantennas.com has templates that will boost the gain of the output of your router. http://freeantennas.com/projects/template2/index.html
This one actually works pretty good.
Hope this helps,