1. Everyone wants to be missed.
2. Everyone wants to be happily surprised.
I actively try to make Sarah know I miss her and I'm thinking of her. I want her to know she's my priority, even when I'm away from her working or training or hanging out with the guys. When I get home, I look her in the eye and tell her "it's so good to see you". When something in my day makes me think of her, I let her know it. When we're apart, I try to find ways to let her know I'm her #1 fan, either with little notes or a quick text message or an emailed picture.
I also try to surprise her, "sweep her off her feet" in big and little ways. If life were predictable, nothing would have value. There's no excitement, no thrill. Replace that with a life where you fill your loved ones with wonder. Instead of getting an "oh that's nice", you get a "how did you know?!" Practically I do this by 1. Learning what Sarah values, then 2. Finding ways to slip those things into her life. Half the fun is the effort you put into it, the other half is the memories you get out of it.
I think that's what "keeping the spark" is all about: Making memorable memories together. You fuel the relationship by missing one another and you give generous, spontaneous gifts to each other.