Love is a verb
Not to take away from any of Hallmark's business, but one of the best ways to communicate to someone how special they are is to send them a card that is blank on the inside and write them a Power Thank You*. It has three parts:
1. Thank them for what they did for you. The more specific and visual you can be in your description, the more emotionally heartfelt you will feel it and they will feel it coming from you.
2. Acknowledge your appreciation of the effort it took for them to do it. How did they go out of their way? Did they stick by you through a particularly rough time? Did they stand up for you when you were under fire? Did they stand up to you and firmly tell you how much them believed in you when you didn't believe in you?
3. Tell them what it personally meant to you. One spouse told me how they said to their partner: "If I have never told you what a blessing you have been to me, it's long overdue, I'm sorry and I'm telling you now. I was emotionally homesick for a home I never had and sick from the one I had. Then I met you and you gave me a home that I never thought I would have or be able to have."