In your view, what single action can one take to make oneself a man after God's heart? What single factor is common to those that have a record of being: God's friends; pleasing to God; favoured by God? SHALOM.

you are attempting to walk through the eye of a needle. a single action will not capture Gods heart. its a plethora of actions

I advice you read the bible, especially new testament. You will find the way to God's heart therein. Yes, David was a man after God's heart but Jesus gave us summary of it all - LOVE.

Believe in the Lord Jesus Christ.

What is in the heart of a man after God's heart? Let's see,
you can read the whole of chapter 2 samuel 24, David sinned and he wanted to rear an altar unto the Lord in threshingfloor of Araunah the Jebusite. For the sacrifice on the altar, Araunah surendered everything to David, look at what David said, 2 samuel 24:24" And the King said unto Araunah, Nay; but i will surely buy it of thee at a price: neither will i offer burnt offerings unto the LORD my God of that which DOTH COST ME NOTHING. So David bought the threshing floor and the oxen for fifty shekels of silver".
Any thing you can do to help the work of Evangelism, will go a long way to capture God's heart. After that you need to be straight in your relationship with the Lord, not lukewarm, but fervent in spirit serving the Lord.
Humilty is a vital key. I think you also need to study the life of King David from the beginning to the end.
Psalm 138:6 " Though the Lord be high,yet hath he RESPECT unto the LOWLY: but the PROUD he knoweth afar off".
There are other things, as you continue to walk with the Lord, he will order your step.
God bless you. God2man.