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Gnome have their own guidelines: for older Gnome 2: http://developer.gnome.org/hig-book/3.0/ and for new Gnome 3 - http://live.gnome.org/GnomeShell/Design/Guidelines
A better and more general page would be: https://live.gnome.org/Design
and for apps
For a music / media player:
Even more (recent) mockups can be found on https://github.com/gnome-design-team/gnome-mockups
Note that new GNome design seems to break radically with older implementations (Banshee, Rhythmbox, ...), and other OSes.
Thanks. Reading the HIG would be step one, of course. I was really looking for some tips that may not be so easily found in written guidelines. The kind of tips you only get after really using a system for a prolonged period of time.