Well, there's all the typical stuff you'll find here. Launch early, get a few users who really like your product, iterate rapidly. You have to be a lot better than any free competitor because people will choose a free alternative over a premium one if they're equal.
Beyond that, try to think of creative ways to market the site. Going the subscription route is very hard, but if whatever it is you have up your sleeve is compelling enough you can make a lot that way.
