Unfortunately, freelancing is a service. Your main goal should be to go out there and sacrifice yourself so others can make money out of your work. If the work you produce doesn't make money, then you should consider who is at fault. If it's not you, then it really is you - because, you should have been consulting with the client in order to improve the product/service or to advise the client against doing this and ask for referral. This is the only case where integrity can be bought (by sacrificing potential work).
Servicing is all about satisfying the needs of other individuals - in the case of businesses, this is always to make more money. As soon as you grasp this, you will start approaching potential customers with the foresight to tell them you want to make them money, hourly, vs. you want to charge them money, hourly.
I'd say the best preparation would be to save up money in advance, so that you may work a couple of months without getting paid. Although, ideally you should realize that you do need money to start freelancing full time - even if it's not your own. Ideally, you will have been freelancing before taking the plunge and have had quite a bit of freelancing experience already.
In the end, the best way to approach freelancing fulltime is to see it as a business. You will be running a business (as a sole-proprietor) and be working for yourself and charging business rates. I'd also urge you to watch and learn from this: https://vimeo.com/22053820
It's a tough game and you need to be a serious player. Best of luck!
I suggest reading this comment (and ensuing discussion): http://news.ycombinator.com/item?id=3420396
First, ditch the word "freelancer" as you are running a business. The change in mindset is important.
Savings gives you runway and allows you to plan out farther into the future. Try to have at least six months worth of living expenses in the bank.
Network like crazy.
An employee gets approximately 2000 hours of work annually. If you are like me and you can only focus on client work for around 6 hours per day, then 1500 hours will be your ceiling. Subract from that all your downtime (sick time, vacations, dead time between projects, etc.) Set your rate accordingly.
Learn to say no. One bad project can be a disaster for your annual earnings.