I do NOT believe this has anything to do with back links. I believe this has to do with CONTENT on the web page.
I have a web site with very few incoming links. Two nights ago, I discovered that one of the pages is #1, on Bing, for a good long tail keyword. Approximately 2.95 million web pages with those words on them. It is AHEAD of *very* large companies.
Also, I do NOT believe this has to do with keywords. I do put 3 keywords on each web page, however, I believe Google, Bing and Yahoo do not pay ANY attention to them. I believe they are looking at CONTENT.
Try to make it maximum of 3 backlinks on per page. But backlinks is only one face of serach engine more important is quality of contents. If you have rich keywords and original content then it will work more in search engine.
Quantity links doesn't matter. How quality links back from your niche industry. It is the important one. Backlinks are only increase your site traffic strength. Bcoz Google loves how many quality backlinks in your site.
You need to constantly building backlinks to your site because if you won't then your competition might seized the opportunity and outrank you. Thats pretty common in SEO, specially if your niche/market is very competitive i.e hosting, weight loss etc.
That is the best method that you can use to maintain your ranking.