Basically, its just experience. Keep working at it, you'll get better over time. If you like something particular about a site, take a look at the HTML output to see how they did it. Then try and implement it on your own site. Don't just copy it, because of copyright reasons, plus you won't understand how it works that way. One way to make a site look very immature is to have lots of flashing animated GIF's and pointless Javascript (like stuff that follows the mouse) - never do that.
Blair