Do NOT Feed the Trolls: A Rant against Idiocy

1) Quality examples.

There are two kinds of mods - the active posters who lead discussion, and those who do the shadowy punishment. You can't do both at the same time, because it looks bad if you're banning people that are being morons while arguing with you, so you ideally need multiple mods or very good discretion. This part is about the former.

When possible, create good topics, make good posts. Most forums, at any stage of decay, have good posters or people who would be good posters given the incentive, and they tend to stick together and be attracted to posts that aren't garbage. It's essential to have the forum 'veterans' (either mods or normal posters) presenting a good example of what proper posting is, so that newer posters take reference from that. This is why changing a bad forum into a good one is a damned nightmare because you've got the wrong examples in place - so get to work doing your part. You'll also, conversely, need to remove the negative examples of this, but I'll cover that later.

2) Forum presence.

The moderator can't effectively do things on the forum. The admins have the real power, you can only influence or advise them, as well as try to keep order with what abilities you have. As a result, 95% of a moderator's job is getting people to respect what he represents. Therefore, people need to know that the moderator is there. Make posts and announcements, make sure you show up (have a hot pink username if you want), let people know that the boss is around, because you need to build the image that you are authority. This can be done through either quality posting (see above) or mod actions (see below). If people don't keep moderation in their mind, it's easy for them to lose standards if they're not already ingrained.

3) Make examples.

One of the things I love about TL and hated about the forums I controlled was that TL mods actually have power. I assume you have it too. Use it. Warnings are good, but for most troublemakers, a warning is a reason to bitch and then 'get off free'. Ban.

The biggest weapons you have in enforcing a higher quality culture developing is coming down like a meteor on bad posts. 1-3 day bans can hurt if they're used to handle infractions - if people post like trash, ban them and let people know you did it and why. This ties in to the previous two points: when people see shit posts being hammered and that the mod is on the ball, it has an effect in encouraging people to post properly (and getting rid of the trash). Don't just play janitor and remove/edit posts (or if you do anyways, let people know you did it), leave 'USER WAS BANNED' as evidence, TL-style.

You need to direct physical force at the ability of people to post badly, because telling people 'don't post like this' only works if they're an aberration. If everyone posts like garbage, people aren't going to care or know what you mean, so warn with some force behind it to trim (or hew) away the worst examples.

4) Get staffing.

You said you don't have manpower, and that's a problem. You do need people to be available around the clock, as this matters for point 2. When people see bad threads/posts proliferating, they begin to accept that as standard, and thus the faster you react the less it can develop and the more people shy away from such behavior. You're also going to benefit from a group of visible moderators leading/contributing to discussion, as well as others (or the same group in rotation) handling the purges (see point 1). One person can make a difference in such a situation but it's extremely difficult and the results can be limited or temporary.

Draft what good forum veterans there are if they're willing to help (TL did this with banlings, etc), either to 'sub-mod' position or higher, as these members are already highly visible (as they likely have a lot of posts) and thus have good positioning to have an impact. You can bring in people you know to help mod, but be careful: people need to know who they are and that they're reasonable or they won't respect them (for example, I was contacted to mod a forum that I had minor relations with, and had to spend time developing a credible account to take the position).

5) Culture momentum

The last point I'll bring up is about how the forum works. Refer to Mondragon's TL-attack: he doesn't post on SC community sites because they're useless to him. Good posters don't go towards bad sites, bad posters can't maintain themselves on good sites. From what you've described, you're massively in the former (through no fault of your own, perhaps), and thus you need a lot of constant work to push it up, as well as a user transfusion. Remove bad users, get some quality control in, and better posters will be more willing to move in. We see the opposite with TL somewhat - with SC2, a huge wave of lower-quality posters arrive, and are able to lower the quality enough to survive (while before a single poster would have likely been eaten alive by Manifesto), because you can't simply ban half the forum. In your case, if you get a core of good posters in place, you can re-build up the community around it, and that should be your main focus.