There is nothing more frustrating than working hard to plan on a piece of writing, go through several drafts, and put it into final print, thinking it is perfect, only to deliver it to your client without a payment. You don't think anything of it at first, figuring that they are just taking some time to make sure that it is right, but when you finally see it in print or posted online, you realize that you may have been duped out of your time.
While this isn't the norm for writing online or off, it does happen. Sometimes you will find that clients either forget about you altogether, don't have the funds to pay you, or never intended on giving you money for your work in the first place. This of course can have quite the negative impact on your income, as well as on your motivation to write. The more clients that you have that don't pay you, the more negatively you are going to look at the whole writing business in general. There are ways however to deal with these issues.
When you are dealing online, the best thing is that you can create an electronic trail from proposal, to acceptance of your writing. This means that if you are communicating with a publisher through email, by some sort of message system, or other electronic means, you can create a paper trail that will show that you had an offer to write something for this company or individual, that they offered to pay you a said amount if done by a said date, and then if you see it posted online or if you see it in print, you now have shown that they have deemed this to be an acceptable piece of work. At this point in time, if you have kept all of the information, you have officially made a contract, without either of you ever meaning to do it. This is something that could be easily admissible in court as evidence against them. Of course, sometimes taking someone to court over $20 or $50 might now even be worth the hassle. But if you mention that you have this documentation, then they may be more apt to pay you in order to avoid the chance of ending up in court explaining their situation to a judge, which they would surely lose.
Probably your best however, is to make sure that you create a contract with the person you are doing business with. By creating a contract that establishes what the person is looking for, what the agreed upon rate will be, when it is deliverable, when the approval/rejection will come, and how soon payment will be delivered, you now make the publisher aware that you are serious about getting paid and they will feel more obligated to make sure that you are paid.
What I generally do when I'm freelancing is make sure that I get a good faith payment up front. This means that they will give me a portion of the money, normally not much, up front so that I can be sure that I will at least get something for my efforts. I would much rather get $5 out of the $75 that I'm owed than nothing at all. At least this way I know that it won't be a total loss.
You could also go through a middleman that makes the person purchasing your work place the money aside in a separate account. Once your article has been approved, you will receive the money and they will keep the article.
The big thing to remember is to always stay ahead of those that you are dealing with. Make sure that you check them out to see if any writers have had past issues with them if you don't trust them. And if at all possible try to do work with companies that have paid you in the past, or those that have a great track record of payment.