There are many internet sites devoted to HIIT. Just do a Google. Some offer free information and some will require memberships with access to more, or you can purchase a particular trainer's books.
I do HIIT 3 days a week, sometimes 4. It just depends on what my workouts have been for the week/month. The other 2-3 days I devote to doing a different style, iTread, dvd workouts, yoga practice, etc.