I've given a guide on how to grow and maintain your beard from the positive side; now on the inverse - what are the worst things to do for your beard. Really it comes down to physically touching your beard in a bad way.
Work on adjusting your habits so that you don't focus on one hair, but instead stroke the full beard. I've found that having a comb near by is very handy, and while combing a beard is still a physical activity - it's not as harsh as the picking / plucking.
From there, over washing can cause brittleness, breakage, and split ends. If you need to wash your beard daily; then look into investing in a beard wash + softener that are designed specifically for beard care. Simply using that $1 shampoo from the local drug store won't be good for your beard.
Finally, keep stress down, eat right, and exercise. Do those things and your beard will be looking it's best.