I don't offer specials unless that is made on the spot due to unforeseen circumstances.  If you are shooting a family, individuals or head shots you're building a relationship on the spot or it could be a returning customer.  I can offer a free print or an extra B&W low rez digital for social media as a "gifting" for using my talent.  This builds trust and word-of-mouth references.  Customer references are priceless and brings you business because their friend or family spoke about you.  We live in a society that wants to be unique.  The referenced customer coming in will want more than what they heard or saw making what you have "gifted" always comes back and pays dividends.  Another way to look at this is "selling it forward".  A slight variation of "paying it forward".