Via George Ou's blog: How to embed high quality YouTube videos and more. The methods described there are a little cumbersome (except for installing the Wordpress plugin - but I'm not using Wordpress :-)), so I've simplified them a little. You can either paste the embed tag given by Youtube in the text-box below and press the generate button:
Or use the following bookmarklet: HQ YouTube. All you will have to do is to drag the link on your bookmark bar, and when you are on a Youtube page, click it and it will generate the modified embed tag for you.