If you are banned following are the ways to regain your ranking.
1- Use the disavow tool if you have too many spam links.
To use this tool you should have complete knowledge about your links if you used it in wrong way, you can harm your site, so be confident your site is really banned from google panda or penguin.
If your site decreased ranking , first check the site and confirm what is it. It can be google dance or something else.
2- According to lattes panda and penguin update meta keywords are not so important but adding them is a plus sign, Most of big search engines extract data from meta keywords like bing , yahoo, and alixa.
There are some link percentages introduced after panda update.
3- Use 40% Bound links like “click here” “go here” or something like this. 40% URL links which contain full url’s of your site. and only 20% direct anchor text links.
4- Don’t over optimize the site by keyword stuffing or spam methods ( On page or off page )
5- First check your TF- and IRF values in your content and then select the keywords very carefully. By choosing wrong keywords can harm the site or could be ban.
6- Use only quality content and don’t compromise on it. That’s the key of success.