Making content for PMC is the best way to get subscribers. Skins, Texture Packs, Data Packs, Mods. Generally participating in events is the best way to gain subscribers as these can gather a decent amount of attention official or not. But if you're not good at making things like those you can do some other things on the forums. You can make Polls, Events, Comics, Drawings. You just need to support the community on PMC and make content.