There are so many auto repair shops everywhere you go. How do you, or your customers for that matter, decide where to have your vehicles repaired? Everyone relies on price-per-service, so there has to be something more than that that makes customers come into the shop. It may help to look at the competitors and move on from there.
Free Donuts and Coffee
How many service stations in your area use this ploy? Customers "love them some free donuts," do they not? Coffee and donuts for free really cannot be beat, especially if you have customers coming in in the morning to either wait for simple car jobs, or dropping a vehicle off for something more complex. Maybe instead of free donuts and coffee (or in addition to!) you could offer them free rides to work and pick-up service after they have finished work and when their vehicles are ready. Bet there is not a lot of that going around with other shops!
Loaner cars used to be a really big deal. Customers could take a loaner car off your lot and drive it around until their own vehicles were fixed. The service was also free, which was huge. You could start up this kind of service for customers again. Just make sure you have the cars insured to the hilt to cover the customers and the vehicle in the event that an accident occurs. (Also, make sure you have a very strict driving contract for customers to sign so that all of your bases are covered.)
Punch Cards for Services over $50
Where on earth did this service idea go? Like the loaner cars, there used to be service stations that provided customers with punch cards. All services over $50 received a punch. After so many punches, the customer got "x" amount of dollars off of their next service in excess of $50. That is not too hard to do, either. Buy a set of tires, get a punch. Put in a new transmission, get a punch. Fix the drive train, get a punch.
It not only encourages customers to take better and more timely care of their vehicles, but it also gives customers what they want; something extra for nothing. You could easily drum up new business for your auto repair shop fast with the whole punch card system. All that new business for the punch cards would more than make up for the discounts you are giving.