This is a good question! We've found that by simply playing the music CD's from the different HOD guides that the kiddos review many of their verses that way.

This is because much of the music in the guides has to do with the Scriptures that the kiddos are memorizing.
My sister also always has her kiddos highlight every one of the verses memorized throughout the guides in their own personal Bibles. She starts in LHFHG and as the kiddos' Bibles change and grow with them, they re-highlight in their new Bibles too. In that way, it is easy for them to see at a glance, during Bible study or listening in church, if they come upon a verse they've memorized through HOD in the past.

In the upper guides, from Preparing on up the kiddos memorize entire chapters, so at that point the highlighting is extensive!
It is amazing how much Scripture the kiddos have actually committed to memory as they travel through the HOD guides.
Blessings,
Carrie