It really depends a lot on the relationship. For my wife and I it's really been effortless, though I'm not sure how long the "honeymoon phase" is meant to last. We're only coming on our second anniversary.
The best advice I've ever heard is that your partner should be your best friend. They should be the easiest person to talk to. When something good happens and you want to share it or something bad happens and you need to talk about it, they should be the first one you think of. You shouldn't feel drained by spending time with them (despite normally being an introvert, I don't find myself needing "alone time" where my wife is concerned). You should be able to be happy for no other reason than that they're happy. You should be able to put their happiness before your own when it's important (though you shouldn't tolerate an abusive relationship). You should always put them before anyone else. You should never lie to them, or feel the need to.
Enjoy what you have now. Realize that it's only the beginning of a journey which could be long and wonderful. Think of it as... I don't know, the tutorial level of a video game or something. It's quick, exciting, and over soon, but there's a whole game left that you still have yet to play.
Good relationships won't necessarily always be easy or pleasant. They're sort of an increasing random walk, like the stock market or something.
I've been with the same person for 16 years now. It's honestly been extremely rewarding, but in a way I couldn't have anticipated when it started, or even a few years into it.
My sense is that success in relationships basically come down to two people being a good match (physically and psychologically), both being committed, both communicating, and randomness.
In my own marriage, every year to year-and-a-half, I've looked around and realized that we weren't as close as we used to be. We've had to rebuild the relationship, which takes work. It's been worth it, though - still being in the honeymoon phase at our 20th anniversary was pretty cool.
I mean, yes, the relationship also deepens and grows. The head-over-heels-in-love can last for a long time even while the relationship deepens, though...
The next phases are better. It is the essence of faith that your future will be better than your past. You cant see how,now, but it will. The word confidence will overcome you because of it. Like "The Force"
Buy a ten year journal and jot down stuff. Read it every decade or so.
I read stuff i wrote in 1976 and wonder who wrote it. What was important then isn't now.
Every year gets better- even with things not always going perfectly.
You have a partner for your short time on Earth. As it spins the Earth as roulette wheel keeps randomly picking "a number" to which yours will be chosen.
Continue to make your partnership the highest priority in life and never assume that things have to be worse.