In the latest LJ_news it has been mentioned that you can now look for people's posts in a community by using this nifty code:
http://community.livejournal.com/egl/?poster=laviefantasque [Using my own username as illustration]
So next time you are looking for a post and you remember who but not when, this might help! Just thought I would mention it. I'm also going to post this in daily_lolita where I think this will be especially handy? Enjoy!