Paul__B:
Do correct me/ shout me down if this is not the most appropriate place, or if this has so blatantly been done before,
I am far too polite to shout 
Anyhow: http://arduino.cc/forum/index.php/topic,76140.html Unfortunatly owing to a strange misunderstanding this is not a sticky, so people who answer have to remember to have their own cut-n-paste bookmark file to refer to it. That example was the result of rather lengthy discussion amongst regular contributors where just about every line was revised to be as correct and educational as possible. * MP3 Shield * - Rogue Robotics rMP3 - #4 by bhagman - News - Arduino Forum
There have - of course - been others that have tried to do explain this "how to have several things at once". Many times. There are libraries with "timer classes" so that it hides the complexity (though how using such a library should be easier than remembering 3 lines of code... ? Ah well, we are all different) I've seen at least two attempts to make a new "framework" where loop() has been taken away and people just write sections of code which then get called at the right intervalls.
Despite all that, there will be people that will ask "The Question" - again and again. Partly beacuse it is too difficult to find the refereneces, partly because it presumes they read, think and understand the issue before willy-nilly cut-n-paste code.