Is there a site that keeps track of when volumes of manga are supposed to be published? Or any other easy way of getting that info? I've been using amazon so far, but I think they post the info much later than it becomes available.

Best place you can go is the publication of the manga's site. But there probably is a site that mass lists release dates

actually, what sort of manga are you looking for? Japanese or English one?

Either way, I recommend the publisher's site. In fact, it's better to know the publisher's site to see the expected release day for the manga.

If you are talking about Shuukyuu shoujo or Nyotaika, then at the end of the volume there's always a postscript...I think in Nyotaika, the author said something like: at first he (or she?) intended to do end the story at chapter 12. But then the vol 2 was out...so the story was made longer, the mangaka also said something like maybe he would make a third volume since "there would be people, without a doubt, say something like "that's it? what will they do from now on?", so the mangaka might do a third volume. But then again, he/she admitted that in his personal opinion, 2 volume is quite enough. But we don't know :X. So it's like 30, 40% for Nyotaika for a third volume...

General speaking, you might want to read the Atogaki at the end of every volume

Well, manga in Japanese of course. But yeah, publisher sites are too much work. I was looking for an easy way. But I guess it was not meant to be...