praying for our persecutors

I was reading something a while ago, I can't remember where, that we should pray God's blessing on those people who cause us pain.
The idea was that nothing can happen to us that God doesn't allow, so when we pray for our persecutors we ought to pray that God bless them for what they have done. If they are doing what is necessary for our growth then we should ask that God bless them accordingly. If however, they are sent by the evil one, then by praying that God would bless them we are allowing God, and not us, to bring about punishment.
In either case we leave it up to God to reward in accordance with his will, and in the meantime we do not burden ourselves with anger or hate.
So daily I am trying to practice this. Often its not until after I've had a whinge and a moan that I remember to pray God's blessing on the bearer of pain, but increasingly I am able to bless and not moan at what God is allowing in my life so that I might become the person he wants me to come.
"Lord, you are the only one who knows what is truly happening. You know the beginning from the end. And so I ask that you would bless those people who I have found difficult today. Lord I pray you bless them in accordance with how what they have done has been in accordance with your will and purposes."