Tracking Users - Where they come from, how long they stay and how many buy.

There are many different analytics packages that allow you to track who is coming to your website and what they are doing. By figuring out how many of your visitors become buyers, what keywords draw buyers in, and how much you spend per buyer. Knowing this information can make or break online marketing campaigns.

I recommend: Google Analytics is a free, easy to use interface which tracks users. It also integrates very well with Google AdWords.

If you don't mind paying a bit more, I would recommend ClickTracks or even Crazy Egg.

Optimizing Your Site

Once you know who is coming to your site, you need to do a little testing to see what works and what doesn't. Does that red order button turn customers away? Is your logo too loud? Maybe the 1-800 number is helping.

I recommend: The only way to do this is something called multivariate testing. Multivariate testing allows you to manipulate various elements of your website, and track which ones have the best success at turning visitors into buyers or leads.
Again, Google Analytics can be set up for multivariate testing. Probably the greatest resource is the Marketing Experiments site, which has case studies, multiple vendors, and anything else you can think of.

Developing new revenue streams

You've figured out where people are coming from, you know what makes them buy - but how do you increase your revenue. If you're a content site, you can sell Ads on your site. If you sell products or services, open up an affiliate program. Not only will this boost sales, it will help your SEO process as well, as affiliates will link directly to your site.

I recommend: Selling ads on your site is easy with Adsense
You can sell text links on your site through text-link-ads.
The oldest affiliate network is Commission Junction. Try also ShareASale and Azoogle.