The way to make it back to Google index is to simply send in the reinclusion request. This is how it is supposed to work. However, Barry on Search engine Roundtable informs about a WebmasterWorld thread which states that Google Reinclusion request forces you to 'admit' that you're a spammer.
The post on WebmastersWorld says:
I have gone to Google’s reinclusion request form to look into submitting a reinclusion request for the site. However, to submit such a request, you MUST agree to a declaration that the site has been spamming (you must agree to: “I believe this site has violated Google's quality guidelines in the past.")
So what do I do? I don’t want to sign a declaration that says we’ve been spammers, because it isn’t true. I’m quite happy for people at Google to look into why the domain is not indexed, as I think it’s clean, but I don’t want to be forced to ‘admit’ to being a spammer when that’s not the case.
Do you think that it is reasonable for Google to demand an admission of liability for them to look into what may be an error on their part?
Another member responds to the thread:
No. I think it's a rather arrogant way to approach this problem.
Barry says, “There are times where one takes over a domain that may have done something against Google's guidelines. In that case, you can easily say, yes – it is possible, the previous owner, did things that were wrong. But if not?”
Join the forums on WebmastersWorld to give your take.