Getting and staying flexible is probably one of the most important things you can do to have and maintain good karate. It's good to arrive at the dojo a few minutes early and stretch. After class is over, you should take a few moments to stretch again.
When you're at home, watching TV, sit on the floor and stretch.
Stretch all parts of your body as often as you can.
Don't over stretch, though.
Here's a good article I found with some accompanying videos on stretching...