The most important thing is to make the API design not only easy to use, but also slightly defensive. If people should doing things one way, guide them toward that in the API, and make it harder for them to do the wrong thing. A lot of documentation can be made redundant in this manner.
As for the documentation itself, I know of no good way to put yourself in the shoes of someone who does not know your API - you are just too familiar with it to be a good judge of the documentation. Instead, test it. Get others to read it and try to accomplish particular tasks, invite feedback (and be open to it - if they get something wrong it's not their fault, it's yours, they're the experts in how useful this documentation is so listen to them), and iterate.
As for the documentation itself, I know of no good way to put yourself in the shoes of someone who does not know your API - you are just too familiar with it to be a good judge of the documentation. Instead, test it. Get others to read it and try to accomplish particular tasks, invite feedback (and be open to it - if they get something wrong it's not their fault, it's yours, they're the experts in how useful this documentation is so listen to them), and iterate.
Good luck!