Successful prayer is based upon God's Word. We must ask in accordance to His Known Will and Word.
1 John 5:14-15: "And this is the confidence (the assurance, the privilege of boldness) which we have in Him: We are sure, that if we ask anything (make any request) according to His will (in agreement with His own plan), He listens to and hears us. And since we [positively] know that He listens to us in whatever we ask, we also know [with settled and absolute knowledge] that we have [granted us as our present possessions] the requests made of Him." (AMPC version)