What is the best way to create a songs database that is independent from a service?
Each group has their own song database automatically. Each song you add to a presentation is automatically added to your group's song database. The songs are keyed by the song title. When you're entering a new song title you will see suggestions from your song database that match the entered title prefix.
I have also seen users create a "Song Database" presentation which is simply a presentation comprised completely of songs. This gives you the added flexibility of associating a unique background with each song. Then you can use copy/paste or the Reuse Items feature to select songs to add to your presentation.
Interesting idea...from Powerpoint days, I have three HUGE Powerpoint presentations with song slides. It had the added benefit of being a place I could search if I couldn't think of a song that would fit with a particular message. It would be a MASSIVE Proclaim file, though...