My wife and I got together when I was 17 and she was 20; we got married six years later, in 1981, and we're still married. So that makes... what? 37 years, or 43, depending on how you count.
It's hard to say anything categorical about how to invest in your marriage. It depends on who you both are and what you both need. For us the critical investment was designated time together -- a day of the week that's dedicated to hanging out together, and usually to getting outdoors together. "Going out and looking at a tree" is what we call it. Of course sometimes it ends up being something really exciting like going out and getting our flu shots together. Life impinges. But we usually get out to look at our tree. Usually we make love, or at least read aloud and cuddle, on that day.
But as I say, that's us and what we need. At critical junctures marriage counseling has been a godsend.
The main piece of advice I've got is: ask, and ask again. Ask what's going on with your partner. Ask what they want that they're not getting. Ask what they like in bed (it's scarcely credible to me, but I have it on good authority that many people actually don't know what their partners like. It's your job to know this. It's not necessarily your job to deliver it, but you really need to know it.) Keep asking and listening. It's easy to think you know what's going on, but you always know less than you think you do. Always. And the only way to find out is to ask and to listen. Practice follow-up questions like "tell me more about that?" And "what's that like?" Because generally the first time you ask, people just say what they think you want to hear, or what you should hear. It's the second or third ask that actually starts to unearth information :-)
Try to keep the asking and finding out separate from negotiating solutions. Solutions will emerge if you know what the problems actually are. But conduct your fact-finding missions first, before you try to fix anything.
