Just adding the fmt=18 bit to the URLs in a video’s regular embed code won’t work. You have to add a couple of small pieces elsewhere, too. Here are the changes to make:

• value="(VIDEO EMBED URL)&ap=%2526fmt%3D18"
• src="(VIDEO EMBED URL)&ap=%2526fmt%3D18"

http://howto.wired.com/wiki/Watch_Higher_Quality_YouTube_Videos#Embedding