I don't pick at my scalp, but I do pull my hair out - or did, until 17 months ago. Trichotillomania (compulsive hair pulling) is an impulse control disorder, and I think skin picking might be as well.

What worked for me is becoming more aware of when I was pulling my hair out, and also trying to avoid temptation wherever possible. You could try writing down each time you pick at your scalp and what you're feeling at the time, to see if there's anything that triggers it. (For me, it's when I'm bored, or worrying about something.) Avoiding temptation is the hard one - I try and keep my hands busy so I don't have one spare to go to my head, and if that's not enough I'll put on a hat or tie my hair back.

An important thing to remember is that when you're trying to stop doing something like this, it's absolutely fine if you relapse! Just try not to let it discourage you. This has got to be the fourth time at least that I've tried to quit, and it's been by far the most successful attempt.

Hope I helped a little, and good luck!