The first is the simplest. When the backup begins, simply click the small "x" that appears in iTunes' main window. This will halt the backup but continue the sync, so those new podcasts you've downloaded will get transferred.
The second procedure is a bit more involved. Open a Terminal window and type the following:
defaults write com.apple.iTunes DeviceBackupsDisabled -bool true
That will kill the backup procedure until you enable it again by typing
defaults write com.apple.iTunes DeviceBackupsDisabled -bool false
Just be aware that this method also kills SMS message backups.
Those of you uncomfortable with Terminal can use this tiny application which essentially runs the commands above at the click of a button.
Finally, you can follow the steps outlined at iPhoneFreak that involve disabling the option to send diagnostic information to Apple at sync.
So there you have it -- three ways to make your backup procedure more pleasant. If you need to recover files from a backup, you can use the mdhelper application, written by our own Erica Sadun.
Update: The DisableBackup application in method #2 above has been updated to take advantage of a new feature of iTunes 7.7.1. Details here.