How to set default input source in Mac OS X Lion

I use two keyboard layouts (input sources) in my everyday work - the standard US and Russian-PC. In System Preferences/Language & Text/Input Sources I have activated the "Allow different input source for each document" option. Incidentally, I also have English as the top language in System Pereferences/Language & Text/Languages, even though that is not relevant to this particular problem. I also have Input Menu enabled at login window and experience no problems with that.

However, when launching a new window or application, I can never be sure what language I will start typing in. That's very inconvenient, especially in full-screen apps/windows/modes. I have been unable to determine a consistent pattern of when and why the input source changes or whether it even consistently "sticks" to the an application or window.

Extensive googling and searching through existing q&a here yielded no satisfactory answers. Most confuse the question with the language selection. I have once stumbled on a tip saying that if you press-and-hold the input source switch keyboard shortcut (⌘-space in my case), you will get a visual switch cue (which is true), and that the default will "stick" to the value at which you release the keys (which I couldn't prove is working, at least for me, also default for what? current window?)

So the question is - how do I set a systemwide default input source, which will be applied to every new invoked application and its windows?

I asked Seth from CNET about this, and he replied me this article (Managing input methods in OS X, I'm reading it, wish it will help us!! I use Sougou Input, Chinese Traditional, English and Deutsch, this whole changing game makes me crazy!!

To expand on Han's answer, here is the excerpt from the article Managing input methods in OS X

To change your default input method in OS X and avoid having to make these adjustments for each application and document you open, you first have to be sure the setting to allow a different input source for each document is disabled. To do this, go to the Input Sources system preferences and select the option to "Use the same one in all documents." With this setting in place, go to the input menu and select the desired method you would like to have as your primary input source. After this is done, you can go back to allowing a different input source for each document, and any newly opened documents and applications will now use the desired primary input method, until you manually change it for that document.