If you are up for some technical work you could replace the standard WRT54g firmware with a "enhanced" version from the internet. The one I used both here and at home is a version called "ddwrt". Using this firmware allows you to do a lot more with the router as well as boosting the signal.
This firmware isn't the only one available but it is one of the best. Be aware that there is a possibility of damaging the router if you perform the upgrade incorrectly. The instructions with the firmware go through all of this though.
Take a look at
http://en.wikipedia.org/wiki/WRT54G under the section Third Party firmware projects.
Another trick is to use a CD to hang off the antenna (shiny side up) on the router to more or less guide the signal to where you want it.