Apparently there is little fix for this. The primary culprit is your ISP skipping around on their IP address -- which screws up your session ID. If you have AOL, it could be a problem.
And I've been getting these messages, too. I am so used to them, though, I just hit "Submit" on my posts a second time to push them through and don't think about it. Only have one bulletin board window open at a time in Internet Explorer and you might see this message less often, too.