I think the best way is just by doing. You start with an idea in mind, for example a metallic logo for a website , or a wood background texture for an iphone app, or rounded border gradient button. So look for tutorials in google, you will find a lot, so look for one you like the final result and make it.

By doing tutorials you will learn a lot of tricks and concepts on how to combine layers, styles, create shapes, etc... at the same time you will get work done (with better or worse results but done) Then becoming a pixel perfect designer it's just a matter of practice, and by combining the tricks and concepts you will learn in the tutorials, you will be able to do whatever you may want.

You will find nice tutorials in places like:

http://psd.tutsplus.com/

http://www.smashingmagazine.com/