I've recently had a couple breakthroughs in my graphic design abilities. I didn't go to art school or have any design training growing up, so the aspect of web design has been the biggest challenge for me.
Some principles that have given me a boost lately:
1) Study letterhead design for Mastheads, color schemes, layout, typography and even navigation!
2) Don't get hooked on learning CSS to the exclusion of all else. Knowing CSS is important, but it won't make me a better designer. It will only give me power over layout. It's important to spend equal time studying graphic design.
3) Color! Recognize great color schemes on the web or in print. If you see something on the web, grab the colors using a color utility, or copy the graphic to your hard drive and bring it into Photoshop/Illustrator to make a color pallette from it. Scan printed material and do the same.
4) Illustrator! Learn the basics of Illustrator and use it to design logos, text graphics, etc. That crisp, clean vectoring makes your designs look sharp.
Here's a link to a layout I played with today on a lazy sunday that is based on a letterhead:
I scanned the letterhead to grab that nice mint green color. It is a very subtle green that provides excellent contast for white text.