What's the most effective ways to slim down or remain in shapes?

I've heard that HIIT [High Intensity Interval Training] is very effective. Sprint for 30sec and rest or jog slowly for 90sec then all over again for a few sets. Half an hour is more than enough per session/day.

As you get stronger you can decrease the resting time to 60sec and eventually 30secs.

For the girls, cons are you might grow muscles on your leg.

And of course diet is as important as the training.

I've very big thighs and calves already cos I was super sporty last time and I developed muscles. Kinda regretted )):