I always try to incorporate music into youth activities and/or lessons... especially if I can locate the perfect song that deals with the topic I happen to be teaching.
It's not too different than what occurs during the Sunday service. There are many topical songs that are sung during Communion, Christmas, and Easter. When the senior pastor preaches a sermon about forgiveness, the songs may be about how God has forgiven us and how we also need to forgive. The backs of many hymnals even have songs categorized so you can easily select them.
I haven't been able to find a topical music directory available for youth pastors to access (free or subscription-based). Sure, there are a lot of music reviews and recommendations... but I couldn't find a single topical index of individual songs for use in youth ministry.
Any advice, suggestions, or questions concerning this section are encouraged and welcome.