I know everyone is going to describe building something people want or building something that is easy to use or excellent. And this, of course, is the best strategy as it encourages word of mouth marketing (free and effective) and has a wonderful product to fall back on (i.e. it isn't vaporware).

However, something else you should really focus on is bloggers. I don't me spamming them or anything like that. I mean determining exactly what benefits your product offers and who would care about it. Chances are there's a lot of good blogs and bloggers out there that will. Way before launching you should contact them. If they are writing about something that interests you or you think you have something to offer email them or comment. Build a relationship, just make sure it's not forced - anyone can tell if you're being fake. Make sure you're providing value of some sort to them. Then when you have your product or site or whatever run it by them. Even if they don't end up writing about it the fact that they have a decent blog is indicative of their willingness to express their opinion and offer articulate feedback.