It's rarely easy, but you will get better with practice.
Preparing -- even for a few minutes -- helps tremendously. Otherwise, get in the habit of thinking of a simple lede or headline as you report. You'll tweak it as new information comes in, but the process gets you ready to write.
One trick that works for me is to write a bare bones version of the story without looking at my notes. I'll flesh out the details and add quotes from my notes on the second run-though. That way I don't get bogged down in details at the start.
Here's some other advice on deadline writing from the pros.