When you are working hard, writing, trying to become a writer, you tend not to think about what happens once you actually become, a writer. What do you do if you start having money come in? What if you make it big?
Its a big concern. Do NOT spend lavishly, do not show off by spending money. Do, take half of your money, and invest it (CAREFULLY), do take 10% and BLOW IT, have fun, go nuts, you've earned it and you deserve it. Use the rest to pay bills, buy supplies, whatever you really need.
Then, get back to work. Work harder now. You are almost there. Making one big sale, or starting to get an income, does not mean you've made it. If you are still doing it, five or ten years down the road, then you can look back and think, "Yes, I've made it!" But knowing you "made it" is a hindsight kind of thing. If you make it a now thing, or a foresight thing, then you are setting yourself up for failure. Or worse. Like the Lotto winner, who has nothing, is suddenly rich, then suddenly broke, and suddenly massively in debt, worse than ever before; do not, end up like that. Be paranoid, be wary, aware, careful and thoughtful. Set your self up for success not only in your writing, but also, and obviously and most importantly, in your budget.
After all, why are you working to become a writer? Because of the Art of it? The catharsis? The money? If you're doing it with the money, well, get another career. You'd best be doing it for the Art, the process. Then, you cannot fail. If you are doing it for the money, when you finally get some, be sure it stays. So, don't blow it all; invest it; and invest what you've achieved by continuing: write more, divest (money and effort).
If you are screenwriting, write a novel. If you are a novelist, start a blog (get a readership, use ads, make money off it), or better yet, start a blog first, its great practice and good for discipline. Obviously, I have a blog. I do two articles a day, try to write them as far beforehand as possible, edit whenever I can, and my more thoughtful writing has only gotten better and faster from it.
A blog is a bit of work. You don't have the time to perfect it, unless that's all you do, or your have an assistant, or editor, but few of us have that luxury.
I was just reading Script magazine, an article called Mo' Money Mo'. I use Final Draft screenwriting software and it comes with their great magazine, "Script" which I keep a yearly subscription to ever since.There was a great article this latest edition about a beginning screenwriter's finances once you make that first sale. I can't offer the article but here's the link to the Nov/Dec edition: (http://www.scriptmag.com/2010/11/04/whats-new-in-the-novdec-issue-of-script/)
I can offer you this article from Script, however: 8 Financial Tips for Writers.
Best of luck!