I do the following:
1) Right click on the method and click “View call hierarchy” (or shortcut Ctrl+K, Ctrl+T)
2) Expand the “Implements x” folder which will then show you all the implementations of that method. Click on one to go there.
Relatively quick and easy. Annoyingly though there doesn’t seem to be an equivalent for the interface itself.
update: as of Visual Studio 2015 update 1, right click on a method and select go to implementation. You can also map it to keyboard shortcut via Tools > Options > Environment > Keyboard and search for Edit.GoToImplementation command. The default shortcut is Ctrl+F12. (F12 will navigate to the interface).