Means of grace are instruments that God uses to convert and bless people, like the reading and preaching of the word, prayer, singing (psalms, hymns, and spiritual songs; Acts 16:25; Ephesians
5:19; Colossians 3:16), enjoying the beauty of God's creation, baptism, and the Lord's supper.
"The “means of grace” that the Bible talks about are His Word (“the word of His grace,” Acts 20:32), His Spirit (“the Spirit of grace,” Hebrews 10:29), prayer made to the “throne of grace” (Hebrews
4:16), and the grace that is given to the saints which enables them to build up another (Ephesians 4:7; 1 Peter 4:10)." - Bob Deffinbaugh 
- "There is a kind of paradox here: On the one hand I say, I need you. God has appointed you as a means of grace to help me endure to the end. But on the other hand, I must say that the only way you can really help me is by saying something or doing
something that will cause me to depend on God and not you." -John Piper 
- "Breathing is the means that God uses to sustain life. So the command to breathe is the command to fall in with the purposes and patterns of God to give and sustain life. This is what I mean by the term, 'means of grace.' 'Grace'
is the free keeping-work of God to sustain our spiritual life that leads to everlasting joy. The "means of grace" is our 'keeping ourselves in the love of God.' God's 'keeping' inspires and sustains our 'keeping.'
His keeping is decisive and our keeping is dependent on his." -John Piper