Paged posts?

I am trying to cut my posts to 2-3 parts. meaning: after 20 lines, i want to have a “more” link, that goes to the next part of the post.
I’m using the more tag in my posts, but the problem is it is using the first page as an excerpt – when you click the “more” link, it shows the whole post, instead of just the second part.

How do i get the result I’m looking for?
If there’s a way to limit posts to X lines or words automatically, and then show the rest with a “more” link, thats even better, although using the more tag manually is also fine…

Thanks in advance.