One of the interesting things about design is that theory tends to be too slow as a mechanism for accomplishing it in the real world. While you could work everything out from first principles every time you wanted to place a piece of text on a sheet of paper, you'd get out-competed by the guy who spends all day composing layouts and can just "feel" it all.
Natural skill helps, but it's practice and feedback that gives one the ability to reliably and consistently produce work that has the effect you want.
I've just finished laying out a poster of sorts for Cinemon. Nothing extravagent, just some text and a few background graphics. Looking at it now, I can see lots of problems. It's too crowded, too dark, and has too few colours (it's a very conservative design) to support the complexity of the graphics used, and there isn't enough of a difference in visual weight among some of the elements. All choices that would have been made better with a little more practice at graphic design (or actually applying theory, rather than just running full-tilt forward to get the project done ASAP).
Rest of the day was spent on getting the SVG graphics working on the Firefox 1.1 SVG-enabled previews, and then fixing a minor bug I introduced into the URL-generation mechanism.