"and also, in ESB, the empire saw that the millenium falcon was heading for bespin because they put a tracker on it. but when leia, lando and chewie escape they don't remove it (as far as I know) so the empire should have found them again after a while."

I know this was posted a while back, but here's a possible explanation anyway: The tracking beacon was being tracked by a specific tracking device which was on the Death Star. When the DS was destroyed, so was the device. OR they removed it after they landed. The whole tracking device thing is a bit stupid really, Leia KNEW they were being tracked but decided to lead the Empire straight to the rebel base anyway?!? Couldn't the have landed somewhere else and changed ships, or searched for the device and removed it, or rendevoused (sp?) with another rebel ship and transferred the plans to them and led the Imperials off in the wrong direction?!?