As far as I know, there's no way to save a draft without posting it public.
But a lot of people who have a lot to post in a trip report just do it in more than one post, ending non-final posts with something like "more to come". Maybe not quite as dramatic as the "ta-daaa!" of posting a full-grown trip report...but who cares?
I have really become lazy when it comes to trip reports. But when I was doing them, I would write them in a Word document. At the same time I saved the photos I wanted to use and renamed them with numbers. Then when I did the actual trip report, it was pretty much cut and paste. Gave me time to work out the details and edit it before anyone else saw it.