I have read a fair amount of the research on naturally increasing your HGH. There were three things that I found that are legit:
Eating a diet with a low glycemic index (paleo) and never spiking your blood sugar.
Consuming protein (without carbs) prior to sleeping.
HIIT (sprints) as you described.
Edit: HGH is a pretty cool hormone. Its natural release pretty much correlates with being alpha as fuck. In maintaining its release you have to exercise the self-discipline to not over indulge in biological inclination (sugar), but must choose instead to invest in your health as a person.
Throughout our evolutionary past, which constitutes 99% of our history, in order to get meat you had to kill to get it. You were required to apply yourself to successfully take life so that you may prosper. This means being willing to take a risk, and doing what is necessary to ensure your objectives are realized.
Finally, you basically have to be all in. When you work out/ exercise with intensity and take very short (like 30 second) breaks, you are telling your brain and body that it needs to keep up with your uncompromising will, and it responds by releasing HGH.
I'm not trying to be a purist on the subject, but basically, you have to earn naturally released HGH. We only deserve and we will only acquire what we earn and obtain for ourselves.