Personally I've been with my wife for 15 years. Two things about staying with the same person and not getting bored.

1. Sex is about emotional intimacy, not just sex. So it helps to keep that excitement up by ensuring you are emotionally intimate first. Connect first over a play date (could be movies or dinner or ... wrestle ... seriously, wrestling get the adrenaline going and both of you panting) then be sure you let things build up with snuggling, foreplay, etc.
2. Change things up Don't always do the same thing over and over. Try new positions, take a trip to the "toy" store and buy that thing that was always catching your eye, change the location (kitchen, backyard in the hot tub, back of the car on a back road ...), write each other dirty letters, you get the picture. If it's always the rub here, flip over, touch there ... thing over and and over, it'll get boring quick.