When my hubby does something for us, it can be as small as loading the dishwasher, I thank him. I often also tell him he's doing a great job. He does the same for me.
It may seem silly, two married people thanking each other for everyday tasks (after 15 happy years together), until you think about what we're doing.
We're showing appreciation. We're telling the other person 'hey, I noticed. I noticed and although you didn't have to do that, you did and I appreciate it.' If you listen to Dr. Phil (and I don't suggest you do that often, his shows are usually a real downer), you'll hear appreciation come up time and time again as a reason for relationship issues. We don't appreciate our spouse. We don't appreciate our parents. We don't appreciate ourselves.
By showing appreciation, we are also making a mental note. It is easy to 'forget' all the great things our significant others do for us. By verbalizing the thank you, we're more likely to remember. I've caught myself thinking 'I seem to be constantly thanking the hubby today' then I realize (usually moments later) 'hey, he's really working hard.' I know, I can be a bit slow at times, not always making the connection.
I figure that if a complete stranger warrants a thank you, a loved one deserves at least that for doing the same thing.
Oh, yeah, rereading this, I realized I used another 'trick' for a happy relationship. When we talk about our time together, we always say 'happy' years. Has every minute of every day been happy? Of course not. But by saying 'happy' years, it automatically conditions our brains to think of them as happy years. Words are powerful folks. Use them to create the world you want to live in.