The following quotes are from Spurgeon on Prayer, a compilation of culled gems from Charles Spurgeon, which I highly recommend it to anyone.
You must expect to feel weakest when you are enjoying your greatest triumph.
If [God] has said much about prayer, it is because He knows we have much need of it.
A prayerless soul is a Christless soul.
When God seems to close His ear, we must not therefore close our mouths, but rather cry with more earnestness.
If you would reach to something higher than ordinary groveling experience, look to the rock that is higher than you (Ps. 62), and gaze with the eye of faith through the window of importunate prayer. When you open the window on your side, it will not be bolted on the other.
If ever one of woman born might have lived without prayer, it was our spotless, perfect Lord, and yet none was ever so much in supplication as He!
God loves to see the hearts of His people glowing towards Himself. Let us give to God our hearts, all blazing with love, and seek His grace, that the fire may never be quenched, for it will not burn if the Lord does not keep it burning.