I go out of my way to find a relevant thread before beginning a new one; the trick is how to find it!
I have used the forum's hierarchical structure (up-screen, select [Home]), looking for the thread 'where it belongs'. This will work well for some things (the Old UK Postcards thread is in World Wide Postcards) but is not as helpful for others (covers postmarked Scranton).
I have used the in-site search engine (up-screen, select [Search]). I find this is most useful when I am trying to find a thread that was active in the past few weeks/months, and have a suitable keyword ('Scranton', as opposed to 'postcard').
I have also searched from outside the site, googling (for example) 'stamp community old uk postcards'. Google (dot com) is a lot better a picking-up phrases; the in-site search engine will simply return every forum topic that ever included 'old' or 'uk' or 'postcards'.
There are two really good reasons not to bother with any of this.
One is that Good Forum Hygiene and Good Forum Citizenship take time. Instead of searching for the best place to put your post, you could be clicking!
The other is that a new topic appears in both the [View Last 100 New Topics] and [View Last 100 Active Topics] lists (bottom of your screen) and, if you are trying to draw attention to your topic, a new topic will appear on the home page for some days, whereas a newly active topic appears on that list for a matter of hours.
More liberal use of the [Topic Subscription] feature will help in two ways.
One is that you will receive an update if another diligent individual does the right thing, putting new information on an old topic where it belongs.
The other is that you can bring-up your own profile, and have a look at your own subscribed topics, when all you remember is that there was a relevant topic, but not its name.