Since I'm in the process of switching to Ubuntu I've been hunting down tips and tricks. One great resource is the Ubuntu Blog. Today they had a great tip for copying and pasting in GNOME.
Basically, what you do is highlight the text in one application (for example Firefox), switch to another application and then click the middle mouse button where you want the text to show up. Your highlighted text is now exactly where you want it. If you don't feel like using the mouse, you can use Shift+Insert in the second application to paste the text.