We have an internal asp.net forms app where one page is essentially a WYSIWYG editor that users enter 3-5 paragraph reports into. What are the best options for an autosave
This is a perfect task which could be solved by using AJAX. You could set some sort of timer to run that starts as soon as the user makes a change client-side and have it then do an asynchronous call client-side back to your server to send the contents of the WYSIWYG box. You could then disable the timer until the user starts editing again to avoid un-needed asynchronous calls.
This would certainly avoid interrupting the user with a postback.
There are lots of ways to do this but the following MSDN link would be a good start:
http://msdn.microsoft.com/en-us/library/bb398785.aspx
Essentially, that gives you an overview but the main bit you need to focus on is exposing web-services to JavaScript:
http://msdn.microsoft.com/en-us/library/bb398998.aspx