Put yourself about. You won't learn by sticking to people you know, so be confident about joining groups - you can help them as much as they can help you. Speak, but not too much.
Listen. Others have needs too - help them to achieve their goals and they'll open up. They may even pass on an invaluable nugget of information.
Pay attention. If someone's talking to you, don't let your eyes dart around looking for someone better. Fidgeting and saying 'Mm-hmm' won't impress the person who's busy battering you with figures.
Disengage graciously. Getting stuck with the wrong person can be a nuisance, but don't be rude about slipping away. Bring someone else into your conversation before making a break.
Remember names. Swap cards, and discreetly jot names down if you have to. If you do forget, admit it quickly. The alternative - a night of mumbling 'Oh, hello there' - is far more awkward.
Don't bug anybody. If your must-meet contact is in demand, get someone to introduce you. Offer your card, ask the best way to contact them, then withdraw - better that than hassling them with a muddled pitch.