Try to hold the same expression for the entire scan, especially at the beginning. The most common bluriness in a scan is at the corners of the mouth when a smile changes to a relaxed expression. Some expressions are harder to keep than others, so start with a simple smile and work your way up.
The first 3 seconds are what matters most. That's where we capture 90% of the details of your expression. Be electric for 3 seconds. After that, just turn your head with roughly the same expression. You can relax. Just don't talk. Or laugh. You can do both when you're done.
When previewing your scan, we really only need your face to be clear -- don't worry about the top of your head or your ears. It's okay for them to be blurry - we're focusing on that good-looking mug of yours. Feel free to do as many scans as you like and only use the ones that turn out well. Big expressions make for fun figurines!