Sorry, but you already said it in your post.
"Use your own words" is exactly what this lady says.
I would think the most important thing would be to be sincere (as sincere as it's possible to be, on what is, after all pretty much a commercially sponsored tatfest) - which means NOT getting instructions what to write from anybody else - because that wouldn't be coming from you, then, would it?
I've no idea what kind of relationship you're in, but I would have thought most women prefer simple and heartfelt to witty one-liners. Not that there's no place for wit in a relationship - on the contrary. But if it's the one day of the year when you ever make a declaration, I'm sure it's nicer if it's real, and not jokey (unless it's someone you barely know, but fancy your chances - in which case a joke might be OK to break the ice).
Anyway, if it's a true "Valentine", why are you signing it? I thought you were not supposed to sign with anything more revealing than: "Guess who?" ;)