The following is an excerpt from a book I read while I was at the Prayer Mountain in June of 1991. I wrote them in my journal but forgot to write down the source or reference. If my recollection serves me right, I believe the author is E.M. Bounds (but it can also be D.L. Moody or A.W. Tozer)
Persistent praying never faints or grows weary. It is never discouraged. It never yields to cowardice but is lifted up and sustained by a hope that knows no despair and a faith that will not let go. Persistent praying has patience to wait and strength to continue. It never prepares itself to quit praying and refuses to get up from its knees until an answer is received.
Persistence is made up of intensity, perseverance and patience. The apparent delay ins answering prayers is the ground and demand of persistence.