Keeping your legs together when possible and not "spreading eagle" when you sit down are both good examples of body language a good sissy should try to emulate, but they're also very obvious. A sissy pretending to be a man doesn't have much of disguise if she folds one leg over the other and always keeps her elbows at her sides. And although a sissy should never really hide her inner girl, it's an inevitable fact that many do.
But there is a gesture that can be very subtle and very feminine: the limp wrist.
A limp wrist shows submissiveness and docility. Female executives are taught to expunge this gesture from their repertoire since it implies defenselessness and weakness. Its use by women is so commonplace that the term "limp-wrist" is often associated with gay men striving for femininity - but that's not applicable here, sissy. You are a sissy realizing her feminine nature.
Your goal for the next week is to consciously do what subconsciously comes to many women: have limp wrists (until it becomes second nature to you too, sissy). If at all possible your wrists should be bent. Your fingers should never be aligned with your forearm; the back of your wrist should not form a line with the back of your forearm. If you're laying your hands on your lap make sure your wrists are slightly bent. Unless of course you're doing something that can only be done with straight wrists (such as typing).
After a week your muscle memory is sure to have gotten in tune with your feminine self and you should be limp whenever it's appropriate.
As a computer-savvy sissy (you are visiting this blog, after all), you can always say you're bending your wrists as physical exercise to alleviate carpal tunnel syndrome...and to reinforce your femininity.