Is there any way, via a script or anything, that I can recycle forum IDs? For example, say I had a forum ID 12 (.php?fid=12) and I deleted it, could I somehow use that ID for another forum?
After I delete forums for contests or whatever, I don't want the numbers to be so high like .php?fid=55
if it's an empty forum, you could probably just change the fid directly in the database
however, by doing so, you can still mess things up for people. for example when people sign up for a rss feed of a specific forum, that's identified by fid. if you give this fid now to an entirely different forum, their feed will change to something entirely different rather than something that would make it obvious to them that the forum they originally subscribed to no longer exists.
Also all threads and posts are bound to the fid of the forum they are in, so changing this id for forums that have contents is not a good idea, as you'd have to change the ids everywhere.
In the end, during the lifetime of a forum, the id numbers will just become ever larger, that's just how it works. You can get rid of the id in URLs though, using my Google SEO plugin. It gives URLs names instead, i.e. Forum-Name or Thread-Subject. As long as the name or subject is unique...