This is from my own personal experience...
Call the collection agency and tell them you filed bankruptcy and it was discharged in month/year. (assuming the debt occurred before you filed and were discharged). Tell them they made an error and to remove it from your credit. (Be nice though so they will help you) They will ask for your case number. And thats it. Even if the debt was not included in the bk they only verify that in fact you were discharged.
Collection accounts are bought and sold so you will probably have collections still show up occasionally from debts that were discharged through your bk.
I had a few collections, that were included in my bk and that popped up after my bk, removed from my credit by doing this.