How do you rekindle the passion you felt at the beginning of a relationship?

My boyfriend and I have been together for 3.5 years. I feel like we've gotten so comfortable and casual now that there's no longer any excitement.

I always enjoy spending time with him, but it's that initial passion and intensity that's gone missing. I fear I'll never get that back, that maybe relationships just slip into this comfortable zone and there's no way to awaken that romance again.

Is there any hope? What do you think would work?