You can't rank up (intentionally!). At some point, you are chosen. I was a regular member for 2 years. I tried to be helpful in the relevant Latin sections of the forum etc.. Then someone, out of the blue, promoted me to Civis Illustris. It was a total surprise, I had no idea how it had happened. Almost 10 years later, when the forum started to have technical difficulties and after I'd given an occasional technical help to others and shown some skill in the area, I was chosen for a technical admin which I am now. And then there were others who became spam moderators or some dedicated-fora moderators without ever applying for the job... It is completely out of your hands! But usually you know for what you're being rewarded. So, just be a good member, good student and be patient! Latin is an old for-2000-years-unchanging language... a few years mean nothing in its lifespan: we as students have to be patient as well with everything connected to it! Patience & hard work!
In addition to Godmy's excellent summary above, there are things you can do to prevent promotion happening: spamming, breaking forum rules, pretending to be different members on different accounts, harassment, initiating or propagating flame wars. You know the kind of antisocial behaviour I mean.