Just for reference it's
<!--nextpage--> It's actually for Pages, not Posts. Do note that it is theme dependant though. A lot of themes don't include suppot for it. I know engtech has done a review of the different themes. Not sure if he included this in it though. If not, I'm sure Sulz and/ or Timethief can tell you which ones support it. :)
For Posts, you can use the
<!--more--> tag. You have a button for that on your Write Posts page. It depends on the editor that you're using though. With the normal editor, it's a button that says 'more'. With the rich text editor, it's icon #13 on this display. You use it as a break in your posts. What comes before is displayed on your home page followed by a "Read more..." link. When your visitor clicks on that link, they get the entire article.
Hope this helps,