Ok...an odd request. Although personally I'd rather avoid the strikes to the face.
If you want to toughen up the skin of your face among other areas, all you have to do is become an Iron Man Triathaloner [IMG]http://www.fightingarts.com/forums/ubb/smile.gif[/IMG]
No it won't keep you from bleeding, but if you look at the long time practioners their skin takes on an almost leather, weathered type appearance. This is due to the skin increasing its layering in order to deal with the mass exposure to the elements. All you need to due is begin running 70 miles per week...bike 115 miles per week...swim at least 3 times a week for about 3 miles each time and as the years pass you'll see your skin thicken. Of course this will severely cut into your martial art training time...but it will toughen your skin.
On a more serious note though...you are aware that boxers when they're knocked out...that this results in minor bleeding within the brain? Regardless of your answer...I suggest you follow everyone elses advice and train on protecting your head and avoid being hit there. You'll never toughen your face up enough to prevent someone from breaking your skin.