I'll be married nine years next June 17th. During all that time, there hasn't been a single day where I haven't said "I love you" to my spouse -- several times if at all possible. The two facts are related.

Other short phrases which also occasionally come in handy: "I'm sorry," "You're right," "I'll get that" and "Of course I'll go down to the freezer and get you some ice cream, even though it's 3am and you woke me from a dead sleep. There's nothing I'd rather do." Okay, so that last one is not that short. Think about all the times you're entirely unreasonable, and then go get the ice cream.