I think it is very important to have a variation in people. You need to have guests from a variety of age groups, experience levels and sectors of work.
I also think that an ability to casually introduce people of similar interests is a real skill.
Try and work the room and ensure that if person A has loads in common with person B that they are introduced.
This way both people will come away feeling the event was a great success.
Finally, ALWAYS CARRY BUSINESS CARDS. There is nothing worse than being caught short without your details.