In Galatians chapter three St Paul claims that miracles are performed not by the works of the law but by the hearing of faith.  In the above text the evil spirit would not respond to the seven sons of Sceva because they apparently did not have the faith and character of Jesus and Paul.  It seems obvious, therefore, that merely repeating a formula of prayer will not yield a good success rate when it comes to praying for the sick.  It requires people who have been born again, filled with the Spirit, and anointed to cast out evil spirits to command sickness to depart by the hearing of faith.  If my life has the backing of almighty God, then I can speak with authority to the mountain, to the sickness, and by the hearing of faith know that the illness or spirit must obey my command.  The simple prayer of faith will bring healing to the person afflicted.  When people can recognize your anointing you do not have to ask if you can pray with them, they will ask you in faith.  It is not recorded once in scripture where Jesus asked if he could pray for someone, what he would ask is, “what would you like me to do for you?”  The anointing of Jesus coupled with the faith of the person needing healing brought about the cure of the sick.  When your life testifies to your relationship with God, people will see it and respond in faith.  It is in that atmosphere that most of Jesus miracles took place.  As you grow in your relationship with God, as you learn how to speak to any and every difficulty with authority and the hearing of faith, people will recognize your anointing and miracles will begin to take place with a high success rate, then the evil spirits will have to obey your faith and anointing and your reputation will advance the kingdom of God.