I work via:

Elance
Payment terms will be agreed upon up front and 50% should be placed in escrow.   Milestones, or steps, for the project  can be created and weekly reports will be submitted.

oDesk
Most of my oDesk work is charged hourly, letting you fix the number of hours I am allowed to bill each week.  My hourly rate is between $15 and $50 per hour, depending on the nature of your project. (e.g a simple article or product description may be on the lower end of the scale,  while in depth SEO analysis and blog promotion will be higher.)  A payment method has to be verified on your account.

Retainer
If you have an ongoing project, you can pay me a retainer to ensure that I always have time in my schedule to stay on top of your project.  Goals and requirements are outlined at the start, and a fixed fee can be paid up front or in installments until the project ends.   Payment can be made via Paypal or BACS bank transfer.

Single Project
Just want to hire me one job at a time?  No problem.  After agreeing on a fee, what work is to be done and the deadline payment can be made via Paypal or Google Checkout.  50% deposit is due before work begins, with the remainder due within 3 days of completion.

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Depending on the project,  I may charge per hour, per word or per project.   The one constant is that you will always have a clear idea of the budget before the project starts, and there are no hidden charges.  I am happy to say that over 90% of my clients re-hire me for other work.