I am traveling and using a 3G mobile internet provider, it has limit of 3GB download every 7 days. So I was wondering what tweaks can I apply in Ubuntu 12.04.3 to minimize internet usage. I already disabled flash and images on my browser, but what else could I do. Are there some programs that use bandwidth on the background I can disable or any other tips?
Turning off flash and images was a good idea. You may also want to check out a text-only browser like lynx or elinks.
Turn off automatic updates and checks for automatic updates:
Open Software Center, navigate to Edit->Software Sources.... On the Updates tab change Automatically check for updates to Never.
Disable all syncing services you have (Ubuntu One, Dropbox, etc.).