Joshua Bloch has a good presentation outline called How to Design a Good API and Why it Matters. Joshua is a Principal Software Engineer for Google, before that an architect in Sun's Core Java Platform Group (he designed several of the standard java API's). Josh also wrote Effective Java.

Here are a few points from the presentation that I found interesting...

• If you program, you are an API designer
• If you keep the spec short, it?s easy to modify
• API Should Do One Thing and Do it Well
• API Should Be As Small As Possible But No Smaller
• Names Matter - API is a Little Language
• Fail Fast - Report Errors as Soon as Possible After They Occur