While schools and companies furnished with desks and chairs are the norm today, the Japanese traditionally sat on tatami, either directly or on cushions, in their homes. However, many homes today are completely Western style with no tatami rooms, and an increasing number of young Japanese are unable to sit properly on tatami.
Seiza, sitting on your heels
The proper method of sitting on tatami(Seiza style) is to bend the knees 180 degrees, tuck your calves under your thighs, and sit on your heels. This can be a difficult posture to maintain if you are unaccustomed to it because the constant pressure is likely to make your feet go to sleep. Japanese sit their heels if they do tea ceremony, Judo, Kendo, Karate and so on.
Agura, sitting cross-legged
Another, more comfortable way of sitting is to sit cross-legged, but this sitting-style is usually reserved for men (although some women might sit this way when they are ring slacks).
In fact more and more young women sit cross-legged in these days, but in formal places, they are not expected to do so.
Women would usually go from the formal to an informal sitting posture by shifting their feet to one side so that they are not actually sitting on them.