Good question! There's a nominations thread buried somewhere, wher you can nominate someone to be a mod or put yourself forward to be one (edit: whoops I missed that Pommy posted it above!). We really like having helpers, and once in a while we will decide amongst the current staff to ask three members to be helpers and they can learn about how the modding here happens
We then might ask you back for another helper term, and then perhaps offer a permanent position!
Nominating yourself is a good way to highlight to the staff that you're interested so we can think of you when the time comes to recruit helpers. We like to get people that have a bit of experience on the forum so they know how it operates (it doesnt have to be a huge amount, but not brand new), and they've shown themselves to be mature and responsible on the boards and interested in making it a better place! Modding is great fun on AHS - it's why I've stayed years after loosing track of what Ayu's been up to....