Steve Pavlina writes some common things to remember about starting your own company (stupid mistakes made by the newly self employed). As I read through these I realize there is more to them than meets the eye. Steve has been a "non-employee" for 14 years and provides the Cliff's Notes version of real experience.
Things such as "spending too much money" or "spending too little money" are true but should be taken as guidelines rather than law. As with many things there are many shades of grey. BUT, these are very good points to remember and at least read over as a reminder.
Update: Also, check out the Freelancer Self-Assessment.