Looks like you hit on a hot topic. As a web/graphic designer myself, I think the best way to improve your ability in that regard is to look at a lot of good designs; meaning, actively seek them out.
As pixeline says, there's not a lot of objective knowledge to be learned (though there is some). It's more about improving your aesthetic eye. If you look at high quality designs all the time, then your tastes will become more refined and your web designs will naturally conform to your acclimated aesthetics.
For instance, I work at an indie metal label, so from time to time I'm called upon to work on band sites, album designs, magazine ads, sticker/clothing/merch designs, etc. So I'm always flipping through metal magazines and looking at ads designed by other people, or admiring the merch designs of other bands, or checking out the sites of other labels.
This not only serves as a source of inspiration when I'm stuck, helps me to gauge my own abilities and find areas for improvement, but it also helps me track the ever-changing trends and fashions in my particular sphere of design. As we all know, fashion is fickle, and people's tastes are always changing. A good designer knows how to stay just ahead of the curve all the time. This means that your designs don't deviate too drastically from accepted aesthetics (otherwise your designs will be rejected by audiences), but you also don't want to employ design elements that are overused and played out.
If you can ride the knife's edge and innovate enough to stand out, but not so much that the audience is unable to accept it, then you will have mastered the art of web design. A good designer can identify emerging trends and capitalize on them, while making it their own by adding their own twist to it.
If you're just starting to venture into web/graphic design, don't be afraid to emulate others and steal good ideas. Don't plagiarize, and give credit where credit is due, but just as making copies of famous drawings/paintings is an essential training technique in figure drawing & painting, so too is emulating quality designs an effective tool in learning graphic design.
Sites like Best Web Gallery and Screenalicious are excellent places to immerse oneself in quality designs and layouts. I would highly recommend scanning through these sites in your free time to flood your mind with examples of good aesthetics.
EDIT: I also want to emphasize that talent is not as much of a factor as most people would think. More often, (interest)
is what people confuse as "talent." If you truly have an interest in something, you will be motivated to immerse yourself in it and practice it. This in turn leads to better ability, and if started at a young age builds confidence, which leads to an ability gap, which leads to more confidence and more interest, which in turn leads to more practice...
I consider myself a decent graphic designer (you can check out my portfolio via my profile link) and an alright artist. And people often comment on how talented I am, but they don't realize that I have literally spent thousands of hours honing my craft. While other kids were out playing with their friends, I was in my room drawing. That's the only reason I excelled in drawing. And when I first started building websites, they looked just as hideous as most teenagers' Myspace pages. So don't get discouraged when you see the work of "talented" designers. They all started from humble beginnings as well.
