Have a plan. This is important in more ways than one. There is the big picture - what do you hope to do at this event? There is also the smaller picture – what will you talk about?
For the first, you want to have an idea of what your goal is before you go to the event. Is it to meet a certain number of people, period? Is it to meet people from a particular company or industry, or who work in a particular field? It helps if you can see an attendee list in advance, but this may not always be possible.
For the second, it should come naturally, but if the event has a certain focus – for example, a networking event with technical professionals, or people who work in insurance – you may want to have your mind ready to especially focus on one particular area. Being prepared for this will help you manage your conversations in the most productive way.
The second also follows from the first. If you have a big picture plan, you know how you want to drive your conversations, especially if you find out quickly that someone you are meeting may not be an ideal connection for you and you want to make sure you meet plenty of other people who might.