The way I did it was doing rasqueado ALL THE TIME. I would practice slow ISAMI, go faster and faster. Though I tried to make it fluent and not a trottiong sound. When you can do it, it will sound pretty weak, but keep practacing. I dont mean on a guitar, LOL. I mean try doing rasqueado on like a metal tube as fast and hard as you can for like a minuet straight. It may not seem hard, but it kills your forearm. Make your rasqueado very tense and slow. When your hand donsent get tired form the the excercise, make your rasqueados fast and tense as you can.
Also I found a trick. If you do a rasqueado on time ISAMI, and immedatly damp the strings, the rasqueado sounds VERY strong and vibrant.