I've been doing the same thing. However cliched it is, just looking around you and drawing what you see is really important when you come to draw from your imagination.
Check out
http://www.scrapbookmanifesto.com - 'cartoons' doesn't really do his work justice, but the pictures he draws from imagination have a great sense of depth and perspective. If you check his retro section, you can see the development of his style over time.
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