You don't need to be insanely active to join, you just need to be helpful, as reverend said. This is because we value quality over quantity. So even if you don't make too many posts, you will still be a potential as long as the posts you make are good, detailed, and help other people out.
Besides, you shouldn't worry about being a regular. Think about how people who repeatedly say that they want to be a mod never get modded, and moderators are usually people who never said they wanted to have the job. I'm not saying that by posting here you're not eligible, but the reasoning is the same: You should be helpful just for the sake of it. The warm feeling you get when someone thanks you after you helped him should be your only reward. Of course, some people are not as kind, but you should feel satisfied just because you know that you do your best :)
Personally, I didn't even know this list existed until the month before I was voted in. And in fact, even if I was insanely active (even if this looks contradicting about what I said early), I didn't thought I'd make it in. And about post quality, just look at my posts. Seriously, look at them. I'm by no means a role model, but you'll notice two things:
1. My posts are usually longer than the average BBSer's. They are usually detailed (despite some of them being rather short), and include a lot of links (I love Wikipedia articles and how pretty much every word is a link to some other place where you can learn more). When posting on this forum, I try to provide links to back up what I say (like the FAQ or the wiki), and/or links to point the user in the right direction. Also, I'll try to provide pictures if necessary, and even when it's not necessary, it can make your posts easier to understand (if someone asks where something is located in a specific page, sometimes posting a picture may be better than just writing a description).
2. I use a lot of text emoticons, but that's another story :P