There may be an easier way to do this...but I normally use the "Search" function located at the top of the site. You have a couple of options, you can search for keywords, or you can search by author. If I'm looking for a post I made say...a month ago, I'd type in my username into the "author" search and put in the approximate time frame (or not, you can leave that blank) and it'll bring up everything I've posted.
The admins also recommend using the search function w/ the "keyword" option before you make a new post. If a thread has already been made about a topic, it's better to revive an old thread than start a new one about the same topic.
Hope that helps and it's not too confusing.