Then He spoke a parable to them, that men always ought to pray and not lose heart, saying: “There was in a certain city a judge who did not fear God nor regard man. Now there was a widow in that city; and she came to him, saying, ‘Get justice for me from my adversary.’ And he would not for a while; but afterward he said within himself, ‘Though I do not fear God nor regard man, yet because this widow troubles me I will avenge her, lest by her continual coming she weary me.’” Luke 18:1-5, NKJV
I've heard this quoted as "proof" that we should continually beseige heaven for our urgent requests. I'd like to remind you first that the judge in the story was a godless man and not a representative of how God really is. Second--God is not hard of hearing or forgetful. We don't have to repeat our requests to God over and over.
Intercessory prayer is often represented as standing in the person's place before God, when, according to Oswald Chambers, it is actually the other way around--we stand in God's place to people. In which case, I suggest your go to God most humbly and request to know His mind and preferences before you begin that type of prayer for anyone.
I strongly recommend you read this blog post: http://www.gospel.com/blog/index.php/2010/12/13/todays-devotional-intercessory-prayer/
When I'm tempted to pray over and over for the same thing for the same person, it's more for my benefit than the person. Not that I can even remotely be the perfect example, what I do now is again mentally transfer the person from my hands to His.
Can you do that?
Prayer: Lord, forgive me when I doubt Your memory or wonder if You heard me the first time. Forgive me for taking back the burden I gave You before. I give it back to You, knowing You know far better than I do what my friends and relatives need. Thank You for being all knowing and all powerful. Amen.