I haven't tried it, but my guess would be to change the date somehow--set it to March 12, 2015, or something. Although I'm not sure that you CAN have an entry like that on LJ, come to think of it. Hmmm. I hope someone else can better help you out!
I've seen other people do it often... it's like an announcement that's always the first post you see, stuck up there no matter how many pages back or forward you jump in the time span. I'll tell you if I find it out...
It looks like all you need to do is put a future date on your post and use the backdate option. Details are here: http://www.livejournal.com/support/faqbrowse.bml?faqid=91
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http://www.livejournal.com/support/faqbrowse.bml?faqid=91