I'm in a VERY LTR (33 years), so I think I can speak to this "keeping the spark alive" thing pretty well:
1. Keep the lines of communication open: if there's something about the relationship that isn't satisfying, let your partner know that (in as kind and loving a fashion as possible); and make sure that street runs both ways.
2. Keep your mind open: if your partner says, "I'd like to try _____", unless it's likely to leave permanent scars or something, give it a try. And if you didn't like it, you can say, "I tried it, I really didn't like it."
3. Sometimes the spark goes out. Have some patience, and give it a chance to come back.