What really works for new niche-sites?

Hi, this question is related to a specific type of site I usually build, being Amazon review sites. I am looking for a method/package I can reuse over and over to bring bring my sites to number 1, specifically backlink-wise.

I say number 1 because I always make sure it's doable, meaning the top 10 sites will be PR0 and usually won't have more than 100 backlinks, in most cases even not more than 20 or 50.

I'll explain what I'm doing now and what usually happens:

1) I create a Wordpress blog
2) I put 2-3 reviews on it, to begin with
3) I put 1 article on it, related to the products (a "buying guide" for example)
4) Install All-in-One SEO
5) Submit Sitemap to Google

Now, after doing this I leave the site sitting for a couple of days. What I keep seeing return is this: The first 1-3 days my site enters google ranging somewhere from position 5-15 for the main keyword, so not bad for new site without backlinks. Then after those first days, it disappears. By disappearing I mean no search results for the main keyword or seconary keywords. Only when you type in exact matches " " it displays results, and sometimes these are even outranked by sites who don't have the exact match.

Ok, at this point I'd like to know 2 things:

1) Why is this happening?

Even though my site has no backlinks at this time, I see plenty of others with no backlinks sitting in the top 10 with less content. If I were to leave it like this, you think it would return by itself? How long would this take (weeks, months,...?)

2) What is the best way to go from there?

Obviously, start backlinking... but what I'm interested in is which backlinks EXACTLY are best for this SPECIFIC case. I want to do this as cheap as possible. Some possibilies:

- start doing backlink yourself: not an option, just don't want to do it
- blast package: 5000+ backlinks of which a small percentage stick and get indexed over time
- limited amount high PR forum profile links package (usually 250-500)
- limited amount of EDU/GOV links package (usually 25-50)

Which one do you think is most effective and why?

Maybe long post but I really want some answers. Try keeping it structured so mentioned which question you're answering if possible.

Thanks, much appreciated