Body language is *extremely* important in flirting. There are no rules, though. The most important thing is to be natural. If you start acting unnaturally in order to 'send a signal' - the signal you will send is that you're a fake person.
In general, you want to be open and attentive to a flirting partner. If you sit with your legs crossed and arms crossed, that shuts them out of your body space. That's generally a very negative signal to your partner. Of course, lots of people naturally cross their legs, so if you start uncrossing your legs and feel uncomfortable, that's even worse!
It is very important for the person you flirt with to feel Paid Attention To. You don't want to seem bored or uninterested at all. So some things people associate with boredom - twirling your hair or tapping your fingers - might be habits you want to break. You want this person to think you really care about them, not that you're bored stiff and hoping to escape as soon as you can.
While there are helpful hints based on what most people think of an action (for example putting your head on the table and snoring is commonly accepted as a sign of boredom!!) remember they are all GUIDELINES. Your flirtation style is about YOU and YOU showing interest. So however it is that you show interest in things you like, that is what you should do!
