Interval iPod

So, the only way I figured out to sync my interval running to my iPod was to manipulate the stop times of the song in iTunes. I've been using the manipulated playlist three days now and it actually helps... Once the song stops I switch to whichever level I'm supposed to be at. The only bad thing is that sometimes I hear a great song and wish it would continue... but it has to switch to the next song for as a trigger. I read somewhere on the web that you can insert a 'ding' but.. i can't figure out how to do it.