There are so many SEO firms around and from my experience I will discuss some of the white hat & Black hat practices.
White hat SEO Agency
This is the best firm to select for your company as they follow the Google equality guide line. Some practices include:
As for Google local Map listing
• The company address – Business location is correct and it is the physical location of the business.
• Company Name – only the company name is written followed by company description. Do not include phone numbers or advertising text or URLs in the company name.
• Business reviews are genuine and more recognised by the Google and hence will give you a better raking.
Design a web site for readers only not for search engine. Examples including: 12SM PPC agency in Los Angeles
• Page contents are valuable and by no means repeated or copied from a different website.
• Paragraphs are rich full of valuable material that visitors may look for and link to this pages.
• Links: links from relevant sites and from a high page rank, also links from blog comments where they add value to blogs content.
• Heading: is used only where it is required.
• Do not use cloaking or sneaky redirects.
• Page title is kept relevant to page content and keywords are not repeated.
Finally I say you need to be cautious when you are selecting an firm and I suggest a white hat Agency is an agency that go by the Google quality guide line and attempts to optimize your website with clean and valuable practices that will keep you away from search engine index elimination. Unlike the black hat Agency that will use short cuts and dirty tricks to give you a short session of high ranking following by search engine index elimination. If your website is eliminated from the search engine index the search engine may not accept you again to be re-included in the index as you will be doubtful of repeating the same tricks. If you wish for your website to be removed from the index then used black hat tactics.