One of the biggest mistakes made by online business owners is not testing everything they do.  Most people throw up a website and start making a little bit of money, then never change it or try to make it bring in even more money.  You should be testing your sites constantly - trying new headlines, new guarantees, new wording in the sales copy, new prices, etc.  You may have a sales letter that makes you $50.00 per day, but with some testing and tracking you could be making $150 per day - with only a little bit of extra effort.  That extra income multiplied by hundreds of days adds up to a tremendous difference in your bottom line - but only if you take the time to test and track everything you do.