If you are interested in making the High Quality (HQ) link appear automatically on YouTube this is what you need to do when using iMovie:

After you finish editing your video in iMovie, go to File > Export.

Export your video at FULL QUALITY.

Next open your file in QuickTime Pro.

Go to File > Export.

Make sure to select “Movie to Quicktime Movie”

Click Options. Select Settings >

Put the slider to a little bit above “High”, click ok.

Now click “Size”, for dimensions click 640×480.

(optional - If your video is standard 4:3, you don’t have to perserve an aspect ratio. If your video is 16:9 (widescreen) you are going to want to checkmark “Perserve aspect ratio using” and select if you want to use Letterbox or Crop.)

Select “Deinterlace Source Video”. Click ok, then Save

Upload it and YouTube should automatically create the watch in high quality link.

This process requires your copy of Quicktime to be already registered.
If you have not registered your Quicktime software (which unlocks the Quicktime Pro bits) you can buy a QT Pro version 7.5 registration code for $29.99 from the Apple site.

Any video on YouTube can be watched in High Quality even if the link isn't present by simply adding &fmt=18 onto the end of the video's URL.

This method was first outlined by Charles Trippy.

Related

How to make a YouTube embeded player play in High Quality