A bit late in the reply, but here's the thing... i been using photoshop (the full Photoshop) for over 10 years now, as a design tool and the later years as my digital dark room.

Photographically speaking, the possibilities are endless... the coolest effects are the ones that are not seen. this is the true power of photoshop.
for the most part I would have to agree with skyman:
learn about the different ways to use layers, selections, masking and adjustment layers and the sky becomes the limit!
If you shoot people, these will help you retouch skin to give your photos that special quality you usually see in magz...

There are quite a few books that contain a lot of nifty tips on how to improve your workflow in Photoshop.