A simple way to make your interval training about 10 times more intense and the least obvious progression that totally blew me away the last two weeks on my new fat loss workout plan was recovery manipulation.
The first 4 cycles on metabolic resistance circuit days, rest periods go from 3-5 minutes to 1-3 minutes to 30 seconds then 15 seconds between circuits so that each cycle your rest periods get shorter.
The last 3 cycles on hybrid days, rest periods go from 1 minute to 30 seconds to 15 seconds so again recovery intervals get shorter every cycle.
I've used this cool training technique to make my interval training workouts more intense before but not the way CST teaches using a pattern of wave loading called 4x7 so that every 4 days your workouts become more intense which in turn boosts fatloss and endurance faster.
You can watch an example of 4x7 in the video. I'm finished with fat loss today and starting a core strength plan next.