I've struggled with this myself, especially since the answer (the long and short version) has changed so much!
The hardest thing for me has been to not sell myself short. It's easy to say something like: "Well, I do a bunch of things--some freelance writing, I have a blog, I tutor people in a bunch of stuff, and I play music sometimes."
Sound impressive? Not terribly. All of those things are true, and if I told you more about all of them, you might find me interesting (maybe
) but it isn't a very good way to make you a target of everyone at the cocktail party. (Or wherever. I don't think I've ever been to a cocktail party.)
So I tend to go for something fairly general that makes people want to ask for more info, like: "I run my own consulting business." Just as short as sweet as the people who say, "I'm a playwright," or "I work at a bank."
It's also a good reminder for me not to sell myself short--I may not be making big bucks doing what I do (helping people get into business school, as an independent tutor and curriculum product developer), but I am good at it, and it is a valuable skill in the marketplace. Just because "what you do" doesn't lend itself to a snappy two-word response doesn't mean it isn't valuable or interesting. (The opposite is more often the case!)
Good luck speed dating--I'd love to hear about it. I've never been, and since I'm not looking right now, I won't soon, but I must say I'm curious as to what kind of experiences people have.