What is your big question? Can you explain it to anyone in 90 seconds? Can you link exactly what you are doing to something anyone would understand as important? Can you give a brief audience the flavor of what exactly your work entails? Can you tailor this talk to the audience? What if getting some funding depended on a great talk in under two minutes?
Students get much more practice on this sort of thing these days. In fact, Rice University has an actual competition for elevator talks, complete with prizes. They call them elevator pitches because the elevator is a place you might have a moment to explain what you do to someone you otherwise would not engage. Being able to communicate in this format is valuable not only because of what it can tell others, but for what it can tell yourself. Have you gotten lost in the details? Is your question important? Are your data likely to address the important question? What else might you be doing?
Try to put yourself in the place of your listener. Have ready a 90 second, a 5 minute, a 10 minute, and longer version of why you are spending your life on this research question. Imagine you are talking to your colleague, your friend, your grandmother, or your legislator. You might even think about turning your pitch into something written for your blog, your webpage, or your newspaper op-ed. Just remember to be clear, honest and not self-deprecating.
Maybe we should get together with another lab group and practice our elevator pitches with each other. At meetings they can be particularly important. Just remember to listen to the elevator pitches of others as often as you give your own. And follow up with questions.