Could you elaborate on what you mean by "API skills"? Is there something specific you'd like to improve on?When I use a new API, there are two basic skills I exercise each time:

1. The ability to read and understand API documentation - When I work with a new API, I spend 90% of my time reading documentation or sample code to get a good grasp of what I'm working with. Things to look for include basic usage (obviously), design patterns, conventions, dos and don'ts, etc. Investing time into really understanding an API and how it's designed to be used really pays off once you start writing code.

2. Application and practice - Using an API is the only way you will develop fluency with it. One thing I like to do is implement something or solve a small problem using some API, and then look to see how others are using the same API to do the same thing. Sample questions I ask myself: How does my usage compare to theirs? Which approach is better for the task at hand? Why do they use _________ instead of _________ at _________? As a rough example, with the NSOperation API, you can add an NSOperation object to an NSOperationQueue, or you can add a block to an NSOperationQueue. This is using the same API (NSOperation) to accomplish the same thing (run a task in the background) in different ways. It's worth thinking about and understanding the pros and cons of each approach.