Look glamorous. Better to overdress than be too casual - it shows you put in effort.
Arrive no more than 15 minutes late. Call or SMS if you're running late.
Bring the right gift. Ideas: a bottle of nice olive oil, a book, champagne. Avoid anything that takes work, like flowers that need to be put in a vase.
Branch out. Aim to talk to everyone there at some point during the evening. A good opening question is 'How do you know the hostess?'
Offer to help. Be proactive - say, 'Let me pour the wine.' If she gives you the go-ahead take charge of filling glasses.