Google is known for maintaining rigorous SEO practices and excluding websites that misbehave.
Besides over-optimization of site content, other reasons for Google exclusion are hidden text, search engine spamming, linking to bad neighbors, hosting illegal content and inter-linking. If you engage in any form of SEO manipulation and attempt to trick search engines, you might find yourself dealing with re-inclusion sooner or later.
When you realize that Google has excluded you, the first step is to find out why. You need to find out what offended them and correct your mistakes. Check for links to bad neighbors and link farms, for keyword stuffing and doorway pages. It is unlikely that you are unaware of your own sins.
Next, contact Google with a request for re-inclusion. After submitting your re-inclusion request, and fixing all your errors, all you can do is patiently for an answer. Though the process of requesting for re-inclusion is pretty simple, there are some tips which can enhance your chances of succeeding.
Accept your errors and correct them
It is a big mistake to contact Google and play innocent. Fix your mistakes before submitting the re-inclusion request. It is foolish to leave your errors unfixed and hope for re-inclusion because it will never happen. Worse still, you will be reducing your chances for success in future.
Another mistake you can make in your request for re-inclusion is to be rude. Don't threaten Google with lawsuits or suggest that you could boycott their Adwords program. The fact is Google have no obligation to give you free traffic.
Read their webmaster's guidelines
It is wise to double-check so as to be sure you are acting according to Google rules and regulations.
Don't spam them
Google gets hundreds of emails and can't answer every mail immediately it has been submitted. Sending Google tens of emails, even courteous ones, will only worsen your situation.
Is this the first time?
If your site gets excluded several times in a year, it gets very suspicious. If it is the first time you are being banned, you can ask for amnesty. But if your site has been banned multiple times, you might not be lucky with re-inclusion requests for the same site.
Assure them it won't happen again
This is vital because if Google thinks you violate their rules too frequently, they might hesitate to re-include you. If it was not your own fault, explain what happened.