There's many things you can do to determine how you can improve a site. Trackers such as Superstats and Hitbox can provide a lot of information. Some of the best information they can provide is browser usage and screen resolution. Knowing what people are using to see your site can be very beneficial for you as you can design more for a certain browser or resolution. Some trackers, like Hitbox, can also tell you the plugins installed on a person's computer - that's also handy if you have a file archive and need to choose between, say, Quicktime and Real Player.
Surveys are great for a site with medium traffic.
If you can get your users to tell us the best and worst aspect of your layout, you can determine what they feel could use improvement and what you should leave alone. If they love the navigation, but think the logo's tacky, it can give you an idea on what to improve upon. Have them use the old site, and then look at the new ideas. See if they find the new one easier or harder to use. If it's harder, chances are it will be harder for others.