If you want to really increase the power of your WiFi signal and are unconcerned about legalities there are any number of Do It Yourself articles on the web for building amplifiers. Simply do a Google search for WiFi Amplifier.
NOTE: If you elect to go this route and you live in an apartment, an urban neighborhood or anywhere other than in a single family dwelling be aware that you will likely make your neighbors very unhappy because you are interfering with their WiFi.
