Keep doing the little things that you did at the beginning of a relationship to show them that you were thinking about them, like a text message or note on the fridge. Listen - really listen - when they talk. Too often we switch off and think of other things. Be open to your partners suggestions and ideas, even if you don't think they will work. You might be surprised. Share a bath - it'll make you feel like kids again. Have a date night, even if you can't afford to eat out. You could just set the table nicely, with candles, etc. and sit and enjoy each other's company.
Find a time to connect daily if not in person, over the phone even if it is calling each other from work to be able to check in. If you are a person of faith, pray together with your partner daily. To keep your marriage healthy remember to schedule times to be alone together and also to have fun. Remember to express yourself with humility and openly, and also giving your spouse a chance to be truly honest and open by providing an attentive and listening ear. A great article on this is :https://www.marriage.com/advice/relationship/15-key-secrets-to-a-successful-marriage/