Write the verse out by punctuation.
This helps you break the verse down into logical sections.
If the phrase is too long until you come to a punctuation then break the long phrase into easier sections (like the 2nd phrase of our example).

Colossians 3:16
Let the word of Christ dwell in you richly in all wisdom;
teaching and admonishing one another
in psalms and hymns and spiritual songs,
singing with grace in your hearts to the Lord.

Read the reference and the verse 5 times fast
Read the reference and the verse 5 times slow
This should help you be able to say the verse without looking at it.

When you are able to quote the verse without looking at it,
Quote it 49 times

Remember: Your memory is a muscle. The more you use it the stronger it gets.