If I sit with good posture I look uptight and it's uncomfortable, if I slouch I look dejected etc, but it's a lot more comfortable.

I just try and mirror the posture of the girl, but they sit cross legged and all sorts. If I've got a table to lean on, I usually go for that option. But when you're in a big ring of stools around a few tiny tables, you're just kind of by yourself.

Solution?

Mr. Fahrenheit

One solution is to stand up instead, which make it easier to move around and mingle, which I see as a criteria to make things happen. If you persist about sitting down, I guess your only two options are to lean back or lean forward. The only thing you need to think about is to have a straight back (especially your lower back) and not being round-shouldered.

If you want to mirror a girls body language, you need to stick with a more "manly version" of a girl's postures. E.g. crossed legs = putting your ankle on your knee. There are men who cross their legs just like women, but I wouldn't recommend it.