Bill Myers explains how to set your YouTube player to play high quality videos by default.
Go to Account link (top right of page) after you login to your YouTube account,
This will take you to My Account/Overview
Click on Playback setup.
You would get three options:
•Choose my video quality dynamically, based on the current connection speed.
•I have a slow connection. Never play higher-quality video.
•I have a fast connection. Always play higher-quality video when it's available.
Select the third one (I have a fast connection)
Then save changes. Do this only if you really have a high speed connection.