I'm not saying it. If you know what I'm talking about, don't put it in the comment section. If you really can't resist, just throw your towel over your keyboard.
The situations are thus:
First: Let the universe--that is, all that exists, in any form--be the world that is here, and some afterlife.
Second: Let the universe be only this one, but reincarnation happens.
Last: Let us assume that this world is all that is. The Earth, the Sun, the planets, the stars all exist. However, that's it. No afterlife, not even reincarnation, simply what is here now, what came before you, and what will continue after you leave. You are in no way a part of it, save in memories and other echoes in people's minds. Maybe in your constructions, if you made any.
Respectively, the most basic reasons for doing good would be:
1)to get into a good afterlife,
2)to make the world livable/better when you come back,
3)no outside reason.
Tell someone who doesn't believe in the first that one is your reason and that's...not a very good impression. It isn't a good impression even if the person does believe: So the only reason you're being good is to get something? Huh. That's not really good, then, is it?
Tell someone who doesn't believe in the second that two is your reason and it's a little better. But it's still selfish. It is doing something because you will get something out of it in the end. Not really a morally upstanding train of logic.
Then there's the last. And the thing is, three is a lovely answer to "Why do you do good?" regardless of what you believe, assuming you are doing good. It may not be very specific, but starting at the basic, it can work. You do not do good for any good outside what you are accomplishing. You do good works for their own sake, because "People are good and I want to help them," or "Because it seems like a nice idea."
Whether you believe you are getting an afterlife, or a reincarnation, or nothing, that last makes sense. Put simply: You may be interested in which of these options are true--or if another one is--but as far as behavior goes, there's almost no reason for it to matter. Because being good is a good thing.*
And really, if whatever entity you're trying to impress would say that's wrong, do you want to follow that being anyway? What, would you follow Cthuhlu?
* Note this doesn't even take trust into account--doing good makes people think that you will probably do good in the future. This can make them more likely to help you. At the very least, if you need/want to hurt someone, are you going after the guy who brings you chocolate every day or the one who is a constant nuisance?