Book a flight in Delta Comfort (W class) for the day which you want to fly on where there's currently an MD-88, whatever the cheapest routing is. On the day of your actual flight, if there's a plane change, use the same-day confirm option ($75, assuming you don't have Delta status) to change onto a flight with an MD-88.
I'm suggesting Comfort because regular economy tickets need the same fare class to be available, which isn't often true the day of the flight. W class is available on the day of flight far more frequently. Combine this with booking an EWR-ATL leg as the previous poster suggested and your chances of ending up on an MD-88 are pretty sound.