6 Answers

When I hear the word "passion", I immediately think physical/sexual; but passion has other facets.  

You could start by being considerate of the other person.  Bring him/her breakfast in bed (or at least coffee and the newspaper),  a love note or card stuck on the bathroom mirror.    Turn off the TV and sit close together, talking about what happened to each other today.  Really listen. Plan an outing that brings you together, not just a movie. 
Role play....the one where you pretend to not know each other and "meet" at a tavern, grocery store, coffee house....

RESPECT

Hope someone here can give you you sound advise. I haven't lost mine (yet).

What passion was there when the relationship was entered into? Think about it and try to re-live it.

Add a third person once in a while, for one

Start a new fire..