If someone asks you "what do you do" the best way to respond is QUICKLY -- short and succinct. Entrepreneurs call it the "elevator pitch". Essentially the idea is to deliver an idea for a product or service in about thirty seconds. But as an attorney you are not talking about an idea, you are talking about you. An even BETTER reason for being brief. Remember that the best conversation, for you, is allowing the other person to do most of the talking.
Here is an idea that might help you boil your "what do you do" statement down to a minimum. Answer the question with only one breath. For most of us that means about 10 - 15 seconds. Anything more is too much. If your listener needs more information, they will ask. If you make your answer interesting, your listener will definitely ask for more.
During the dot com boom days I worked with a securities attorney whom, if asked, "What do you do?" could have responded with a whole lot of information about M&A, venture finance, alliance agreements, etc. Instead he always responded, "I am a business attorney and I specialize in making millionaires." Of course his audience would always be intrigued and ask, "How do you do that?" His response was, "It depends on what my business clients need to succeed. Tell me about where you are right now." Ingenious! By asking the right questions he only needed to talk about the specific areas of expertise that would be of interest to his audience.
Remember -- one breath. Let your audience lead the rest of the discovery process.