I think the best way to keep a relationship from getting boring is to create goals and reach milestones with each other. If you are always reaching for something new together, it is hard to get bored with each other. I've seen couples who invest in a shared interest be very happy in their relationship.
Most relationships are like a roller coaster, they have their high points and low points, and the key is knowing what to do when your relationship gets boring or mundane.
Well I'm not in one, so cannot speak from experience at this point in time Betts89 >Thoughthe key-word that comes to mind is what should be at the forefront of every relationship.. Communication!! If a relationship ship is at the crossroads of no return, then it needs to be addressed while it is yet early days. If left, once those feelings are conjuring up behind the scenes, then no stone left un-turn is neither healthy or beneficial and well no doubt lead to the demise of your once happy relationship and it might be as simple as keeping the gateways of communication open, but for some un-canny reason people begin to close down when they feel discontented with life, the lifestyle they once had become accustomed too has changed and that is not what they signed up for so to speak, so you just got to keep those critical channels open when boredom/mundane feelings arise and find a work around to keep the zest and vitality, even if it means doing new things, boredom can lead to depression, a vicious circle ensues, that can lead to a complete shut down, hence communication ceases.
Actually life will never get boring if we strive to continuously try or learn new things, make new friends,etc. Being the same and stagnant is like death. Newness is growth, it is LIFE. It is a refreshing, engaging.
Find passion in your life. First learn to live a magnificient life that is utterly fulfilling and enriching then seek to have a relationship. And when you have a relationship do not suffocate each other, let there be space. Have a girls night out or a boys night out. Keep your friends, keep your interest. Most of all always find something you both like to do, learn or work on. Find a project to do together, do community service together or simply take up a new hobby.
When we seek to live & give to our community, life become deeply meaningful. When life is fulfilling and meaningful there is no room for boredom.
I had a good friend that claimed he had a goto'no lose' strategy - whenever his relationship got boring he would suggest a threesome. If she got offended, they broke up. If she said yes, he said he would ask her to marry her.