One of the first mistakes people made was not doing enough research on the company that would host their web sites. Once their sites became more popular they where not able to accommodate their needs and they had to move all of my information to a new hosting company which resulted in down time, loss of money, and an unfavorable reputation with some of my users because of the down time. Do your research first.
Another costly mistake was waiting to long to add a Monthly Newsletter to your web sites. You could have had much larger revenues if you would of implemented a newsletter sooner. You will be surprised by how many people actually open, read, and click on newsletter links. Do not put off this easy money stream!