I’ve followed many of the solutions proposed around the web for this issue. None have worked for me. WordPress is of no help. They still haven’t provided a fix for this inexcusable bug in the latest version of WordPress. And if you write them, they will refuse even to point you to the answer (if it exists) on grounds you are not hosted at WordPress, as if that makes any difference.
So the best solution I have found is: use Windows Live Writer. Since I started using it, all of my posting problems have gone away (and it has a lot more features than any WordPress plug-in anyway).