I had very bad reception on my Powerbook until I found this thread on the macNet forum site. A tip submitted by webdiva solved my problem. The solution involves powering the machine down, removing the battery, and pressing a strip along the side of the compartment. Head over to the macnet forums to read all the details.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20021010051644749