Well here on bitcointalk I guess the only method would be to either quote that post or reply when you see it and think you need it later.
Or use one of the services on the internet which saves the whole page as a reference for future use.
Like this one for example :
https://archive.org/web/I've used it a couple of times here when I gave some spammers/useless posters red trust and I was sure that the thread was going to be deleted so I saved it and included it for everyone to see after it.