When you are first in love and the fires of romance burn brightly and strong, it can be amazingly easy to say "I love you", not just once per day, but many times. Later on, well...let's face it, it doesn't flow as easily. The euphoria of being in love can be deceptive. It can make it appear that love should always be easy. My experience in marriage (that true love will be tested and is often difficult), actually helps my wife and I to stay "in love".
One simple formula, if you could call it that, is to always remember to say "I love you" at least daily. Saying it helps both of you to remember it... to re-realize it. And we need to re-realize the commitment we made and the love we have many, many times over, if we intent to keep a lasting and loving, lifetime commitment with our spouse. A small investment, but over the years it adds up big.
Consider that there may be times when you are both worn out by life. You may be tempted to turn on each other and take out your frustrations in unhealthy ways like cold-shouldering or emotional outbursts, or by just staying angry in some fashion or another. But if you have made the commitment to say "I love you" daily, then you can change the entire momentum of your interactions and maybe talk it out, instead of ignoring or blaming.