"It's easier to maintain a fire than it is to reignite a spark!"
Unfortunately the above statement rings true.
Since we can only control ourself we're kind of limited as to what will happen in our relationships. It actually takes BOTH people to turn a relationship around. If one person is happy/content with the way things are you will find yourself stuck in a rut.
Having said that, You can try and spice things up with a naughty voice mail, email, plan an evening of seduction, have a weekend getaway, or possibly make love in a different room in the house, on the floor in front of the fireplace, the shower....etc
The element of surprise helps in the short term.
Making plans to do something special involving a hotel stay every 3 or 4 months, having a "date night" per week even if there is no sex can gradually restore romance and intimacy.
Note: The above suggestions do not guarantee the "fire" will return as an eternal flame.
Usually when there is a reduction in sex there is also a reduction in wet kisses, hugging, snuggling in bed or on the sofa, taking showers/baths together, loving eye contact, flirting, sexual innuendo, hand holding, and touching one another in general. In order to increase sexual activity for the long-term you have to increase intimacy.
"When we change, our circumstances change"
Once we stop doing all the things that won our mate's heart the relationship was bound to change.