If you have control over the router, one free as in beer way is to change the channel. Download iStumbler
to see which channel has the lowest amount of clutter and then switch to it. Depending on how many wi-fi users are around, it can make a big difference in connection quality.
If you don't control a router, then you can still use it to hunt for a better signal if one exists.
Other than that, I'm not aware of any way to boost wi-fi performance on the Powerbook in any significant way. That line of laptops is not known for having good wi-fi reception...
The issue is in the computer's antennae. You could improve things with a PCMCIA or USB add-on adaptor that has an external antennae. But, finding one that works easily with the Mac is a bit of an issue (many require you to hack drivers).