This can be worked in several ways:

a) 50% upfront and the balance on completion – this is the way I like to go with most transcription or copy-editing jobs.  You send me the spec, I send a quote, and if you’re happy, you pay 50% upfront, I complete the work, and the balance is paid. Proof of payment of the first 50% constitutes acceptance of the job. And proof of payment to be send after the final payment is made, too.

b) For a project which goes on over a period of time, and can be broken down into quantifiable segments, we will break the payment down into three, four or more sections – a percentage at the start, a percentage when a third or a quarter is done, and so on.