Forget paid links, and even paid reviews, as well as link exchanges. All of them are either not scalable, or inefficient.

Blog comments can work (check out CommentLuv).

The most efficient way is to build a great product and build connections with bloggers/journalists before you need them. Talk to them about what interests them. Ask them compelling questions. Heck, feed their ego. And when the time comes, mention your site, and tell them to share it with anyone they think would want it.

It's not so easy to explain the whole SEO process in a comment. People wrote books for that. A good starting point is guides by seomoz.org. Blog comments, paid links are not recommended, from comments no link juice passes, and google already announced that the sites having paid links will be penalized. Reciprocal links are also not recommended, from homepage to homepage link exchange can be harmful, google spam team everyday reviews websites with that issue. Best way for the backlinks is having valuable content that people will link back without asking them.

if at all possible, i'd say it's guest blogging / writing. this not only build high quality links but it spurs instant virality when done correctly.