We suggest to shave hair to 0,2-0,5 mm. The best choice is to use a razor blade and shave your hair in the same direction of the growth. In this way you will obtain 0,2 mm and you can cut your hair every 2 days.
In alternative you can use an electric razor like Wahl Balding Clipper (you can see the result on this guy, this razor cuts to 0,5 mm). In this case you should trim every day.
If a person has enough density, he can keep his hair a bit longer.