Rather than try and improve your retention and recall in a limited area, the growing body of research suggests that you should work on this in a more general manner.
For example, the simple act of maintaining a daily journal helps general (and specific) recall, even if you never again read what you have written.
Another tip is to take a five minute break about every 30 or so minutes - just do something else entirely, like play minesweeper or go get a drink. Make sure you do limit it to 5 minutes, though.
One that I've read recently is that if you spend some time reading/listening to something and concentrating on it, then the same amount of time with it in the background, then you will retain more than if you just concentrated for that whole time. Obviously, reading is not easily backgrounded - but the plethora of podcasts and vodcasts make this simpler... you can focus on the reading, and then listen to a podcast with similar content while doing the dishes.
I agree with most of the other answers there which consider learning specific API functions to be trees in the woods... Really what you need to learn are what kinds of trees there are in the wood, rather than specific configurations of branches and leaves.