Since Chrome doesn't allow to enable extensions in incognito mode only, you may use a different user profile for incognito mode, and set up the extensions you need for the incognito mode in this user profile.
In order to do it, create a shortcut on your Desktop with the following command (for Windows XP):
"C:\Documents and Settings\Your Username\Local Settings\Application Data\Google\Chrome\Application\chrome.exe" --user-data-dir="..\User Data\Inco" --incognito
This solution has two drawbacks:
- You have to run the incognito mode via a separate shortcut.
- You have to install and update the extensions for incognito and normal browsing separately.