Wow, have I ever fallen off the bandwagon on this one! But the key is, to just get started again - don't give up! We only have two more verses to go in Psalm 139.
Search me, O God, and know my heart; test me and know my anxious thoughts.
We saw in v1 of this Psalm that God does search the heart, so this was not said for God's sake, but rather for David's.
From Barnes' commentary...
The psalmist had stated the fact that it is a characteristic of God that he "does" search the heart; and he here prays that God "would" exercise that power in relation to himself; that as God could know all that there is within the heart, he would examine him with the closest scrutiny, so that he might be under no delusion or self-deception; that he might not indulge in any false hopes; that he might not cherish any improper feelings or desires. The prayer denotes great "sincerity" on the part of the psalmist. It indicates also self-distrust. It is an expression of what all must feel who have any just views of themselves - that the heart is very corrupt; that we are liable to deceive ourselves; and that the most thorough search "should" be made that we be "not" deceived and lost.....Search me thoroughly; examine not merely my outward conduct, but what I think about; what are my purposes; what passes through my mind; what occupies my imagination and my memory; what secures my affections and controls my will. He must be a very sincere man who prays that God will search his thoughts, for there are few who would be willing that their fellow-men, even their best friends, should know all that they are thinking about.
As you repeat this verse over and over again, may this be your prayer. That God would examine our hearts, thoughts, motives thoroughly and reveal them to us so that we would make Him Lord over everything in our lives and take captive every thought for Him.
LeRoy Eims on the importance of Scripture memory: "I think two of the master strokes of the devil have been to convince people he doesn't exist, and that Scripture memory is only for children. He remembers the humility of defeat when Jesus Christ, who was tempted in all points like as we are, met him with the Word of God." (What Every Christian Should Know About Growing; p. 26-27)