In a guide book for spiritual progression called "Initiation into Hermetics" by Franz Bardon Step one outlines a process of thought control where you start out doing 10 minutes a day of passively observing your thoughts and trying to recollect them at the end of the session, once this can be done with ease you naturally progress into being able to hold a single thought, and afterwards an absence of thought. Personally I can barely do the last for 5 minutes at this point. It takes a lot of hard work and dedication. Also you are suppose to increase the amount of meditation each time you do it to whatever is comfortable. I like doing an hour a day.