Forum Moderators: open
You can either pay for the links/ads or spend much time doing exchanges. Some sites have had considerable success early on but this isn't for everyone and these sites are probably so good anyway, that once a few thousand links are obtained, other sites link automatically as the content is so good.
But basically you need very good content, a great fresh idea that offers a service people badly need, and a viral marketing system. In order for a site to take off, will need a ton of links pointing to it.
I'd say about 5,000ish is okay for starters. Getting the first 300 is quite easy, as directory submissions will do that. Links exchange sites can provide some free exchanges also, but they won't all be targeted.
If you want free links? Then other than the link swap strategy, I dunno. Obviously swapping links is 100% free and that's nice, but you'll find a lot just won't give you the time of day if you're new and not established. Directories give brand awareness, but you actually want ROI - you need cold hard cashflow.