I've never really delved into the Harry Potter side of FF.net, but generally speaking I haven't heard of a better way to promote a fanfic other that posting it in the right place, using the right tags, making your summary interesting, updating regularly, and reviewing other people's work. Sure, there are other things you can do and other places you can post things, but they seem to get a much lower response than doing those five things on FF.net.
Also, people on FF.net can fav you, recommend your stories, and add your work to a C2 archive, all of which people will check up on for new reads. I've still got one story that gets sporadic responses because this, despite not updating in three years and the story only having three chapters. Those aren't things you have any control over though, and isn't something you can explicitly count on.