On my PC the longest part of the sync process in iTunes with my iPhone or iPad is the device backup. Taking up to an hour at times I can live without a backup. Especially since I jailbreak my devices which makes ever restoring a backup after an OS upgrade a bad idea.

There are two methods to disable the device backup feature. First, I'm going to show you the manual tweak.

Manual Method
Apple made it very difficult to disable the automatic device backup feature manually by burying the setting deep in a configuration file. In order to make the change you will need a good XML text editor. I suggest using the free Notepad++ that you can download here.
http://notepad-plus-plus.org/download

First, make sure iTunes is closed and create a backup of the iTunes Preferences file located in C:\Users\Your Username\AppData\Roaming\Apple Computer\iTunes\ called iTunesPrefs.xml.

Open up C:\Users\Your Username\AppData\Roaming\Apple Computer\iTunes\iTunesPrefs.xml in your text editor and go to the <key>User Preferences</key> section.

Then copy and paste the code below just after the <dict> node of the User Preferences section as shown below.

<key>DeviceBackupsDisabled</key>
<data>
dHJ1ZQ==
</data>

Save the file and start iTunes. The device backup should now be skipped when syncing.

Automated Method

If you want a fully automated approach there is a free app called iPhone Backup Switch that you can download from here.
http://microseconds.com/?page_id=395

Just run the utility and when iTunes is closed click the Disable iPhone Backup in iTunes button.