You need to work harder on your deportment. A real woman will instinctively smooth her skirt under her bottom before she sits. This ensures –
a) her skirt does not get wrinkled
b) as much material as possible hides her panties from view.

From now on, you will always perform that feminine sweep under your bottom before sitting, no matter what the situation. Put on a skirt and start practicing until it becomes second nature. Remember, you are to do this every time you sit, even if you are pretending to be a man and wearing trousers.