Ever since I left my day job, I've had a lot of people asking "how did you know you were ready?" or "what can I do to get to that point?" Everyone is a little different in terms of their career trajectory, financial situation, and comfort level with the unknown, but here is what I'd suggest for wouldbe full timers:

1. Test-drive your new gig for a full day. Sure you can write or blog in spurts at home, but are you disciplined enough to do it for longer periods of time with no one looking over your shoulder? I used a few holidays and vacation days to see if I could commit to a full day of writing. Fortunately, I discovered that it hardly feels like work (except when an editor or client asks me for rewrites – that feels like work!).
2. Create your website. These days, if you don't have a website, then you're missing out on a major marketing opportunity. You'll want to do this before you go full time so you can actively market yourself (see #3). I set up a website inexpensively through VistaPrint and found that having an online presence boosts my credibility and gets me assignments.
3. Build your network. Having several clients and editors who give me steady work gave me the confidence to leave my job. The confidence factor is especially important. I also convinced my old boss to hire me as freelancer while they found my replacement. Most people suggest that you establish relationships with several clients so you're not relying on one or two for all of your income.
4. Find people to advise you. I'm fortunate to know several experienced writers who have taken me under their wing and helped me break into higher paying markets. A mentor can also advise you on negotiating your rates and dealing with a difficult client. They might even give you some of their overflow work when business picks up for them. Be sure to thank them. Often.
5. Join professional organizations in your field. Organizations like Media Bistro or Freelance Success give you a way to connect (and sometimes commiserate) with others in your industry. They also provide a social outlet for people who spend their days working from home.
6. Look into medical and dental insurance. I'm fortunate to live in Massachusetts, where I can buy inexpensive health insurance through the Mass. Health Connector. If you live elsewhere, then you might consider joining the Freelancer's Union or buying COBRA. Factor these costs into your monthly expenses so you'll know how much you really need to make to stay solvent.
7. Save up so you'll have a financial cushion. Experts recommend that you save between three and six months of living expenses (some even say you should bank full a year's worth of expenses!). Of course we all like to think that we'll be able to make money right away, but it takes extra time to process those invoices and cut a check compared to working as an employee and getting direct deposit. One website that I wrote for took over a year to pay me. I haven't written for them since.
8. Open a small business account. You'll want to track your business expenses and make sure that you aren't exceeding your income. You can do this easily with a separate bank account (mine is linked to my other account so I can transfer money easily), which also reminds you to treat your enterprise as a business, not a hobby.
9. Order business cards. You're a professional, remember? No excuses for scribbling your email on a napkin or Post-It. I'm amazed by the random places I meet people who hire writers. I also gave a stack of business cards to my mother, because she loves talking up her daughter's business.
10. Set up your workspace. Once you leave your job, you'll want to the pavement and start working. If you take a weekend or evening to stock up on office supplies and organize your files, you won't have to spend time on this later on.

OK, fellow freelancers, who advice would you add? Anything I'm missing?