wear the same perfume every day. make sure that it's in all of the stores, and that perfume ladies use it in the door ways at the mall. make sure that his pillow will smell like you long after you're gone.
hold his pinky finger instead of his whole hand and then, whenever someone makes him a promise, he'll remember your palm on the smallest part of him.
make sure to tell him your favorite movies, and books and songs, too. so that every time he goes to the store, or reads, or turns on the radio... he'll hear you whispering in his ear.
when you go (and you will), leave without a trace and keep him wondering because without an ending, a story lasts forever.