My opinion on the matter of learning design has always been to place a primary focus on the basic elements and principles (see link at bottom).
There are plenty of abstract design theories to study, but when it comes down to actually working, there is no substitute for a mastery of the basics. Paul Rand has some practical thoughts on design in his books.
Overall, I would place an emphasis on learning to articulate the problems you need to solve through design. Design is not something you can learn in the abstract—there can be no effective design without first defining the problem(s) your project presents. I would say design is 90% problem solving and 10% aesthetics.
I've found the best way to learn design is to first understand the basics, then go out and apply them to the designs you encounter. Look at other interface/web designs and ask yourself questions based on the principles you've learned. The key here is to understand the "why" behind the "how".
As for the "how", project-based tutorials are okay, but you should be careful not to get lost in the actual project. You want to be gathering techniques, not just following steps.
Resources
Wikipedia: Elements & Principles of Design http://en.wikipedia.org/wiki/Design_elements_and_principles
Anything you can find by Paul Rand. Some of his books are out of print, but if you're creative you can find them on the web.
Grid Systems In Graphic Design by Josef Muller-Brockmann
http://www.amazon.com/Systems-Graphic-Systeme-Visuele-Gestal...
