When you view a video, right below the fullscreen button is a button to change quality. After noticing that I quickly looked for a way to make it permanent.
To make it permanent, you have to go into your account settings (top right, next to username) and scroll all the way to the bottom right of all links. Next, you click on "Video Playback Quality" and change your settings to how you like it. I have it on the third one (high quality all the time).