Make sure you don't have a bajillion other programs running - shut down everything BUT your browser. Update your video driver if possible (or roll it back to the last good one). Make sure your security programs are updated, and possibly disable them ONLY while you are on YouTube (and don't  click any non-YouTube links while you are using YouTube). Turn down the screen resolution/color depth if possible (if you can do so without making the display too ugly). If you have a desktop JAMMED full of program icons, move everything but the few you use every single day to the Start menu (or where ever else your available programs are stored). Uninstall then reinstall Flash.

 

That's all I can think of at the moment.