I find a good quote about the kind of relationship we have (friendship, family, dating) that will make them all misty eyed, then I sign it and say something somewhat personal about our relationship. Cards are like the one social thing I’m apparently good at.

Also it helps to put birthdays in your online calendar and include an email reminder a week before their birthday.

Edit: Throw a small printed photo in for good measure, people LOVE stuff like that. Just make sure the picture is special and bonus points if it’s something they completely forgot about.