I don't preview my posts, but I know the site is a bit dodge for things like that. Plenty of times I have hit Submit Post and I get the Database Error screen.
In that circumstance I hit the back button and hope it takes me back to the posting screen with my post in tact. Usually that works, the posting screen comes back with my post still there waiting to be posted. Then I block it up and copy it and submit again. If the Database Error keeps happening then I have my post copied and ready to be pasted when the site decides to start working again, not usually longer than a minute at most.
I have learned to copy my long-arse posts before submitting though, just in case. Then if anything fucks up, you can just go back into the thread, reply, and paste everything you wrote before.
Chalk this one off to experience and write the post again, though do check the thread first and make sure it hasn't posted anyway despite the Database Error. That can lead to double posts.