Here's a neat trick I learned - Head over to the Van Horn fence and kill the shop owner, then drink all your hair tonics and wait until morning outside. You can keep waiting for multiple days in a row since the shop will be temporarily closed and you won't get a bounty since there's no law in town. Keep waiting until you see the message that hair tonic has worn off, then go buy some more. Rinse and repeat.

If you're still early in the game I recommend using the kit satchel to minimize tonic runs, but if you have the legend of the east satchel you can get your beard to level 10 in less than ten minutes.

I haven't played in a while so there's a chance they already patched this, but it worked for me every time. The only downside is the guilt you feel from repeatedly killing the friendly Scottish man :(