One of the most important and easy ways to avoid frustrating API docs is to check them periodically to make sure nothing is out of date.

Last year I started using Amazon's Flexible Payment Service (FPS) and their docs were so out of date that they linked me to a PHP library that was over three years old and already deprecated. I spent weeks getting my billing system working and then as soon as it went live I got an email from Amazon telling me that I was using a deprecated library that was being phased out in the next few months.

I also had the same experience with Twilio, although luckily with them the old library didn't work at all so I didn't waste time developing with it.

Updating basic things like links to helper libraries can save your customers countless hours of headaches and frustration.