Generally, business letters include the opening salutation, "Dear Mr./Ms. [last name]" and the closing salutation, "Best Regards" or something similar. However, the appropriate salutation depends on the industry, level of familiarity with the recipient and personal style.
In most situations, the general salutation of "Dear Mr./Ms." is sufficiently professional. When addressing a woman, use the title "Ms." unless it has previously been made clear whether she prefers to be addressed as "Miss" or "Mrs." All common titles, such as "Mr.," "Ms." or "Dr.," should be abbreviated; however, if no common abbreviations exist, always spell the title in its entirety, such as "Captain," "Sister" or "Father."
If you are addressing a more general audience or do not know the recipient's name, avoid using "To Whom It May Concern" and instead use a title such as "Dear Hiring Manager" or "Dear Claims Adjuster." If addressing a company or group as a whole, it is sometimes acceptable to use the organization's name, such as "Dear Retail Store X," though this is sometimes considered slightly informal.
If the recipient has requested that you address her by first name, use the salutation "Dear Joan," but be aware that this is considered far less formal than using the title and last name. Never use the last name alone.