If you are quite young then sex is certainly one of the ways, but as we get older it gets a little harder to maintain emotional love to it's fullest. Doing things like baking their favorite cake, pie, etc., is one, or a favorite dinner. Doing something out of the ordinary (something silly) like having a picnic at a nice well lit populated park in the middle of the night is crazy, but fun. For no reason go up and give them a kiss on the cheek, a quick hug or "the look!" Make them feel that they are the only one you love.
Some men or women may think you are crazy, but keep at it because it's contagious. Boredome brews from doing ordinary things and never trying new things.