As most ppl above me already said:
- regularly updating your addons (maybe even if it's just a toc bump)
- just writing good addons, word of mouth gets it going
- having a "cool looking" options interface if needed; respond to user feedback/tickets
- optionally localizing for different languages
- some addons are very simple with just 1 single feature and can still get lots of users
- upload your addons to both curse and wowinterface, I see you already did that. some kinds of addons are more popular on curse and some more on wowinterface
- having a lot of addons, some users will check out your other addons if they really liked one of your addons
- anticipate with new addons for every new patch, ie the pet battles addons got huge within a few days
- make fun and unusual addons, gets you in the spotlight. more dancing bears with rainbow fireworks
- you can show youtube footage if applicable, curse supports embedding them on the addon page
- make addons for user requests/complaints, browse the reddit/mmochamp/wowhead/bnet forums. I remember p3lim made an addon by reddit request
- some addon authors even set up their own website or facebook page for their addons
AddOns aren't very popular anymore though like they used to be. Like Zork said, subscribers are dwindling and the
golden age is kinda overdue