I probably lost a lot of potential subscribers by chosing to posting all my older comic pages of my comic, POW! Right in the Nostalgia, leading up to my newest pages as before I chose to mirror my comic here on Taptastic, I already had over 569 pages under my belt (counting guest pages and art), and I just figured posting the older stuff first and then getting to my current stuff ASAP would be a good idea.
Just my thoughts for myself here, but otherwise...
I'd definitely say the fastest ways to lose subscribers is defenitely going lenghtly periods of time without updates either a new comic or even just a post to tell people what's up. Life can get pretty hectic, and unless your comic IS your full-time job, it can be difficult to manage all the time. Just letting your audience know what's up is a good idea rather than go weeks without at least a 'peep'.