Many routers have a signal strength setting. They usually range from 75%-125%. Same with most adapters in the computers. Also, if you have a 802.11b/g router and adapter, changing the router and/or the computer from 'auto detect' mode, to G mode will greatly increase signal strength in many cases. (should be in the manual)
Try different frequencies. Often times other networks in the area on the same channel can interfere with yours. Other electronics like cell phones also interfere. Common channels are 1, 6, and 11. Channel 11 tends to have greater range.
Just a few other tips you could do is make sure your router is in a central place in the house so all parts of the house are covered. Any floors, walls or metal objects will greatly decrease the signal.
You could also make something like this for your antenna, (this actually does work)