My bread and butter used to be freelance professional audio. Usually recording mixing and mastering work, but I'd also do location audio. I also sort of haphazardly do photography freelance? I do a lot of concert photography, and I get a decent amount of my shots licensed. So, I'm familiar with being freelance.
Contracts are important, go to a lawyer or legal zoom to have an airtight contract made up.
No matter how much you want it, someone wants it more than you. And no matter how good you are, there's someone better than you. If you fall behind in either category, you'll be left to languish by the wayside.
Be confident and know what you're worth. Don't do spec work, no one takes people who do spec work seriously. I've done spec work, but only after I just moved to a new area and needed some clout with the local scene.
No one cares about you. That sounds crass, but it's true. When you're first starting, no one cares about you. You're on your own, and if you want chances, you have to make them, no one's going to give them to you. You have to knock on every door, and if that door opens even the slightest you gotta make the best impression you can. Everyone thinks they're a writer, so you've got a lot of competition. If you don't have the clout to make people care about you, there's someone out there who's willing to do it for free. You gotta convince people that you're worth paying for.
Pay your self employment taxes. That's important. Keep a log of incoming income and pay your self employment taxes. And don't be shy about business expenses. Do you write? With a computer? Buy a $330 graphics card, write it off as 'computer upgrades'! I've done that, I've probably shouldn't, but meh.
Freelance work in a creative field is tough. We've cultivated a society in which everyone is a special creative snowflake flower, and I'm not sure if you've noticed this, but most 'photographers', and 'writers' out there aren't photographers or writers. They got a like on facebook or deviantart and decided they're going to be that creative dainty snowflake. Most of those people aren't very good, but it's the infinite monkey theorem, y'know? If you have enough people working for free, eventually one of them will, by chance, produce some good work. When people seeking those sorts of services have that pool to cull from, they're going to.
Your competition isn't other people so much as it is people who know they can pay nothing and get something acceptable. You gotta make yourself so good those cheapasses who don't want to pay for this sort of work want to pay you for it. The signal to noise ratio is huge, it's going to take a lot of talent, a lot of clout, and a lot of networking to make it happen.
Also, networking, that last bit, by far the most important. You can be the best in the world at what you do, but if you don't know anyone, you're screwed. Make contacts, lots of them. Shake every hand you can. Go to every local meet related to your field that you can. Get on good with local people in your field so they can defer work to you. A good network is more important than a good skillset.