The only way to become a mod is to be nominated by one of the mods or Mr. SD himself. Potential mods are discussed and debated by existing mods before an appointment is made. Furthermore, anyone who asks or volunteers to become a mod is automatically thrown out of the running. Once you ask to become a mod, your chances of being approached about it are exactly zero.
We've actually had to pass on a couple of otherwise very good candidates because they asked about becoming mods.
Lastly, mods are appointed by need only. We don't just go out and get new mods because a new one hasn't been nominated in a while, we appoint mods when they are needed. This also means that just because a mod may resign doesn't mean that a new mod will be appointed automatically. For example, when ywc resigned, a new mod was not put in her place.