How can you make your neighbor understand in a nice way, thatt they need to clean up their...?

There's not really any way to tell them that where I don't see them taking offense. I'm sure they're aware that their property is filthy anyway; they just don't care to clean it. It's not like you can force them to either (unless, of course, it's somehow legitimately negatively impacting your life beyond the simple displeasure that comes with looking at it). I mean it is THEIR yard after all.