It is this war of attention and prayer on which both life and death of the soul depend. By attention that we keep our prayer safe and therefore we progress: if we do not have attention to keep it clear and we leave it unguarded, then it is inflected by evil thoughts and we become wicked and hopeless.

St. Symeon the New Theologian

If you do good, you must do it only for God. For this reason you must pay no attention to the ingratitude of people. Expect a reward not here, but from the Lord in heaven. If you expect it here — it will be in vain and you will endure deprivation.

St. Ambrose of Optina

To admire the labours of the saints is good; to emulate them wins salvation; but to wish suddenly to imitate their life in every point is unreasonable and impossible.

St. John Climacus

Run from places of sin as from the plague. For when fruit is not present, we have no frequent desire to eat it.

St. John Climacus

Not every "good" deed is actually good, but only that good deed which is done for the sake of God.  The externals of a particular action are not its essence; God alone sees a person's heart.  We need to humble ourselves, realizing that every good deed is mixed with some passion.

St. Nikon of Optina

Glory be to Him, Who never felt the need of our praising Him; yet felt the need as being kind to us, and thirsted as loving us, and asks us to give to Him, and longs to give to us. His fruit was mingled with us men, that in Him we might come near to Him, Who condescended to us. By the Fruit of His stem He grafted us into His Tree.

St. Ephraim of Syria

Do not read [prayers] hurriedly; pay attention to every word and let the meaning of each word enter your heart.... Pay attention to every word,  and let the sense of each word enter into your heart; understand what you are reading and feel what you are understanding....  These two -- understanding and feeling -- have the effect of making prayer fitting and fruitful. 

St Theophan the Recluse 

He is truly the Sower of all that is good, and we are His farm. The whole harvest of spiritual fruits is by Him and from Him. He taught us this when He said, 'Without Me you can do nothing.'

St. Cyril of Alexandria