There are three things that are key to maintaining desire and fire in a longterm relationship.
1. Each person in the relationship is responsible for continuing to make themselves feel sexy. That means they may work out, eat well, engage in a certain hobby, continue to be curious in their work or spend time with friends, whatever it is that makes them feel confident, attractive and sexy in and of themselves. Each person needs to feel that they are sexually attractive. We usually think of it as our partners responsibility to attract us, but it is far more effective for each of us to care for our own sexuality and the confidence that we are desirable.
2. Maintaining novelty in the relationship. This is generally going to be small things over the long-term.
It could be going on a long weekend vacation or a new restaurant. New events and experiences together.
It also involves talking to your partner like they are new person sometimes. Asking about subtle parts of their life that you would ask about if you were on a date, rather than assuming that you know what’s going on in their lives. Asking about their dreams, hopes and fears, or what has been especially satisfying lately can refresh your curiosity and interest in them. It shows them that you not only care, but that you expect that they are growing and changing in ways you want to know about. This helps you see each other with fresh eyes and acknowledge how each of you is always changing and actually evolving into a new version of yourselves. It normalizes growth and change in the relationship, which makes room for each person to be most alive, engaging and therefore attractive.
3. Go toward disagreements. When there is conflict in the relationship, it is essential that it is worked through immediately, so that no one holds onto things. Resentment is the ultimate killer of desire and excitement. The two preceding points will do nothing if this one has not been attended to.
Finally, while we must remember we are in this for the long run. Attending to what we want right now even if it is in a small way, assures that desire isn't entirely prolonged and gives us a sense of hope, anticipation and motivation for creating those bigger servings of passion that fires our desires and attraction toward each other.