If you've been asked to speak without advance notice, every second before you talk is precious. Make the most of that time by using these techniques:

Pause
Silence never sounds as long to the audience as it does to the speaker. Indeed, communication studies have shown that a presenter can be silent for up to four seconds before anyone thinks something is wrong. So, take those seconds to get organized. Take a sip of water. Walk to the front of the room. That time will go unnoticed by the audience and can have a big impact on your talk.

Paraphrase
Before you get into the heart of your speech, restate the question or request in a concise way. This allows you to both clarify your subject matter and take a little more time to think.

Quotes and Facts
We all have a few all-purpose quotes or topic-specific facts up our sleeves. When called upon to speak impromptu, begin your talk with a relevant quote or fact. This technique gets everyone in the audience on the same page (and gives you a bit more time to prepare).