What do you do when your boyfriend/girlfriend becomes complacent?

There's always a stage in every kind of relationship (friendship, work, bf/gf, marriage) where both or one of the individuals becomes complacent. It always happens after you've known each other for years and get "bored" of the same stuff every day.

How do you get your other half to not become complacent? How do you get things back to the way they used to be when you first knew each other?
What do you do when your boyfriend/girlfriend becomes complacent?
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