I've been a member of many forums over the last 10 years and I've seen a lot of them crumble and die, and I've seen a few that have stood against the test of time and remained somewhat lively.
So here's the main problems this forum has
Problem 1. Pointless threads
There is an overabundance of them and there always is on dying forums. When new members and anons come onto this forum for the first time and see the amount of random garbage on the front page, they don't feel compelled to stay. The board needs to be mostly full with engaging threads.
How to fix -
There needs to be a new rule on the forum that a thread must have substance to it, or something that asks a question or starts a discussion about something remotely interesting, and the subject must be something that doesn't already have a number of threads posted about it.
If people complain about their rights to freedom of speech being taken away, then you can create a general chat thread that people can go to post whatever random stuff that they want.
If you implement this rule, then each member may find at least a portion of the threads somewhat interesting and feel like posting in on a few discussions, rather than seeing the odd thread every now and then that's squished next to a bunch of garbage. It will also make the board look far more interesting than it does now, where there's just crap like "should I have toast for dinner?". That sort of post should be on a general chat thread, so all the random stuff isn't clogging up the boards and making it look like a complete mess.
Pointless threads as well as threads from attention seekers including things like "So long you fuckers, I'm going", should be deleted. No one gives a shit if you are leaving, just leave, don't tell us all about it.
2. Threads about other members.
We shouldn't allow threads that target specific members. No one likes coming on to this forum and seeing a bunch of people bitching about each other. It doesn't look good to new members who don't know what the fuck is going on at all.
How to fix -
The general chat thread is again another way to fix this. If you want to whine on about how much of an asshole Luna is, then you can do it in general chat, rather than taking the time to clog the board up with another stupid thread crying about something.