Plug in or out a USB device, that'll wake the PowerBook even when the lid is closed. If you attach a mouse or a keyboard, clicking or pressing a key will also wake it.
If you want to block it from going to sleep at all, you can try putting aluminum foil over the latch, to block the magnet that triggers the sleep. You can also try utilities like NoSleep or Sleepless (do a search on versiontracker.com for those).
Beware though, PowerBooks rely on an open lid to ventilate the CPU properly, and the LCD screen ages much faster if it is subjected to heat, i.e. it'll not last as long if you use your PB a lot with the lid closed.
We've had a lot of threads on this in these forums, do a search on "PowerBook lid closed not sleep" and you'll find some.