How do you work on the 'spark' once it's already gone? Any tips on how to revive it?

Personally, I don't think it is "gone" per se (more like burning low?). Mainly because we often get so comfortable with each other and everything simply becomes a routine. I would suggest picking up a new activity that both are interested in. Learning something new is exciting in itself, and learning together may help you to discover (or rediscover) a side of your partner that you hardly sees now. If not, going on a trip may serve the same purpose as well. :)