I've never been in love myself, but I image it's anything you can do that is either
1) quirky and humourous with a splash of romanticism
or
2) doesn't really matter in the grand scheme of things, but you do it anyways "just cause"
with 1 I've seen, that the other person aknowledges it right away (an Easter chub chub marshmallo that says "I love you") With 2, they really don't find out until later (a lunch secretly packed with your favorite treat)
like squall says, it's gotta be constant (or at least more than just 1 or 2). Otherwise all you did was a favor
As for showing love online, just e-flirt
P.S: These are for the "special someone." Showing love to family/friends requires a different approach, because it's a dolifferent kind of love.