One antenna that has increased in popularity is the windsurfer antenna. It is very simple to do, and it will increase your signal at least 10 percent. You can either attach it to your wireless card antenna or to your router antenna it works either way. I would write an article on how to do it, but instead I found this youtube video that explains how to build it.
You can get the print out model for the antenna from this site: freeantennas.com
click picture to view video!