Non-refundable deposit due at the time the session is booked. No deposit, no shoot. The balance should be paid at the beginning of the appointment. No balance payment, no shoot.
Giving off the cuff discounts is a bad idea. Word of mouth is the fastest, most effective form of marketing, and these people are going to tell others about "what a great deal you gave them." Others will come to expect the same discounts and will be confused as to why the price you quote them isn't the same as what they heard.
To put it simply, your business will not survive if you continue to treat it like a hobby. Chances are your friends and the people who keep bailing on you are not your target market.