This verse reminds me of two things.  The first is a small round picture that used to hang on my grandmother's bedroom wall.  It was a picture of Jesus knocking on the door of a cottage.  My grandmother showed me that there was no doorknob on the door and explained to me that it meant that when Jesus knocks on the door of your heart, you have to open the door from the inside and let him in.  That's an image that has always stayed with me.

The second thing it reminds me of is my pinky finger.  When I first learned to witness from the scriptures I was taught a scripture for each finger, and one for the palm of my hand.  They went like this:
1. thumb--For all have sinned and come short of the glory of God (Romans 3:23).
2. index finger--For the wages of sin is death, but the gift of God is eternal life through Christ Jesus our Lord (Romans 6:23).
3. middle finger--For God so loved the world that He gave His only begotten Son that whosoever believes in Him shall not perish but have eternal life (John 3:16).
4. ring finger--if we confess our sins, He is faithful and just to forgive our sins, and to cleanse us from all unrighteousness (I John 1:9).
5. pinky finger--Behold I stand at the door and knock. If any man hears my voice and opens the door, I will come in to him and sup with him and he with Me (Rev. 3:20).
6. palm--But as many as received Him, to them gave He the power to become the sons of God, even to them who believe on His name (I John 4:15).

It's a good way for children to learn, but even as an adult, it's the way I present the gospel and God's plan of salvation.  It's great because the last verse puts you right in the palm of His hand!