When you log in there is a little box with a question saying, 'Remember Me?' Tick that. I found that, even if you do get logged out, it remembers you and the post you were trying to send.
If you are doing a really long post which you definitely don't want to lose, you could do it on 'notepad' and then copy and paste it.
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Pegasus
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“Everybody is a genius. But if you judge a fish by it's ability to climb a tree, it will live it's whole life believing that it is stupid.” - Albert Einstein