Sir Abe Olandres on his post “Is an SEO Contest the best place to experiment?” gave two scenarios regarding a contestant’s site on Google SERP:

Scenario 1: You rank currently #10. You add more backlinks, twice more than you currently have. The following day, you’re down to #12. Was it because you linked from a bad neighborhood? Was it because 2 other sites below you added 3 times more backlinks than you did during the shuffle? Was it because you haven’t updated your entry during those times? Or maybe because one entry on #20 jumped to #5 because it got a PR8 link?

Scenario 2: You’re nowhere in the SERPs today. You bought several links from TLA. You edited your meta keywords and description as well as installing Google Sitemaps in your site. The following day, you’re #7. Which one was it –tags, links, sitemaps? The next day, you’re up to #5 without doing anything. You thought it could be the links so you buy more. Boom, you went down to #7 after that. Why? Could it be someone at #6 changed his permalink structure pulling it down and pushing you to #5. The after he got re-indexed, he went back up again. And all along you thought it was them bought links.

And the question is: Why is it that a site goes down or up on the SERP?

Actually, there is only one answer on this question: It’s competition.Every site exist on the web are competing to top on the SERP on any Search Engines.

• Sign up for PayPal and start accepting credit card payments instantly.
  As the world's number one online payment service, PayPal is the fastest way to open your doors to over 150 million member accounts worldwide.
• Promote your product to high quality, targeted websites and blogs.
  Find effective, influential blogs and highly targeted audiences to advertise.Choose to display your ad across entire blog networks to maximize your exposure to a wide audience.