What need then, someone will say, that we should ask God for what we need? Does He not know already what we need? Why then should we pray?
God does indeed know what things we need, and with generosity provides all we need for the refreshment of our bodies, and since He is good He sends down His rains upon the just and the unjust alike, and causes His sun to shine upon the good and the bad (Mt. 5:45), even before we ask Him.
But faith, and the power of virtue, and the kingdom of heaven, these you will not receive unless you ask for them in labouring and steadfastness. We must first long for these things.
Then when you desire them, you must strive with all your heart to obtain them, seeking them with a sincere heart, with patience, and with faith, not being condemned by your conscience, as praying without attention or without reverence, and so in time, when God wills, you will obtain your request.
For He knows better than you when these things are expedient for you. And perhaps He is delaying in giving them to you, designing to keep your attention fixed upon Him; and also that you may know that this is a gift of God, and may safeguard with fear what is given to you.
[...] Do not then lose heart if you do not speedily obtain your request. For if it were known to Our Good Master that were you at once to receive this favour that you would not lose it, He would have been prepared to give it to you unasked. But being concerned for you, He does not do this.
[...] Keeping this in mind, let us continue to give thanks to the Lord whether we receive speedily or slowly that which we pray for. For all things whatsoever the Lord may do He orders all to the end of our salvation; only let us not through faintheartedness cease from our prayers.
It was because of this the Lord spoke the parable of the Widow who persuaded the judge through her steadfastness (Luke 18:2-5): that we also through our steadfastness in prayer may obtain what we ask for.
By this we also show our faith, and our love of God, since though we do not quickly receive what we ask for, yet we remain steadfast in praising Him and giving thanks. Then let us give Him thanks at all times, so that we may be found worthy of receiving His everlasting gifts; since to Him all praise and glory is due for ever and ever. Amen.
Basil the Great (330-379): Monastic Constitutions, ch. 1, 6-7 @ Lectionary Central.