I'm using the preprint2 style, and my abstract is too long to fit on the title page. How do I break it to a new page?

Make sure that you have AASTeX v5.2 or later and then use the "longabstract" option when you invoke AASTeX:

\documentclass[preprint2,longabstract]{aastex}

This option should only be used in preprint2 mode and only when the abstract is too long to fit on the title page. Using it otherwise will have unpredictable consequences.