If you get jobs through sites like freelancer.com, always read feedback on a potential client before agreeing to do work for them. If there are multiple complaints about the client being late with payments or stiffing writers altogether, you're better off waiting for a better opportunity.
When I started out, I ended up doing hundreds of dollars worth of work for a magazine that didn't pay me a cent. They didn't even send me a copy of the magazine. I had to go to a local news stand and buy a copy so I would have a tangible record of all my hard work. Had I paid attention to the complaints about this client I wouldn't have taken the job.
Fortunately, freelancer.com has gone to a milestone and escrow system that helps prevent these problems, and if a client tries to work with you outside the site in order to get around them, it should be a red flag. Yes, it's a pain to give up fees to freelancer (and to PayPal) in exchange for safety, but it's a good idea until you've established that a client is honest and reliable.
When you're working directly with a client outside a freelancing site, make your policies about billing and payments crystal clear. Keep track of your work in a spreadsheet or Word document to submit for payment so there is no confusion about what they owe you. And if they're consistently late paying or make partial payments, at some point you'll have to decide if they're worth keeping as a client.
It's hard to give up any work when you're just getting started, but bad clients can prevent you from building up a list of good clients, and you owe it to yourself to be firm with clients who don't take you or your work seriously enough to pay you in full and on time.
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