Although I always use broadband with unlimited plan, this maybe helpful for people using dongle, limited use broadbands, data card and 2G, 3G SIM based mobile internet.
1)If you are using gmail that run in Firefox or Chrome then you is losing lots of internet data as gmail consume huge mb of data when I open it using the above mentioned browser.
I know Firefox is the user friendly browser and 80% of internet user making use of it. So friends to save internet data install Agent Switcher for firefox and select browse as Internet explorer 6 from the addon.
2) Do not open the same resource on webpage always Download things like PDFs.
3) Always keep on your cache and cookies. So that when next time you load the page which uses same images, texts and preferences. These resources will be fetched from local computer(Literally No data usage for these). Cookies are used by websites which will store your preferences information and enhance your website experience by choosing already set preferences by you. For example, when I use BookMyShow, my cookies save that earlier I had set Jabalpur as my current city and thus this will load Jabalpur's Movie shows and not the default landing page of BookMyShow.
4) Use email client (Example Mozilla Thunderbird, outlook) to check emails and compose emails instead of the web client. Web client consume unnecessary data in showing ads, loading new images, etc
5) you can download Jabber, gtalk, yahoo chatting client, skype instead of their web page version for chatting.For example, Use of yahoo chatting client instead of chatting in web gmail version is will consume less data.
6) Use of Command line web browser in Linux. Lynx (web browser). I believe this greatly reduces the data usage, as by default it does not loads any images(Configurable). CLI based use of browser uses less data than GUI based.