Free-draining gravel backfill against the wall, filter fabric between gravel and soil, a perforated drain pipe at the base that daylights downhill, and weep holes through the face. Never native clay packed against the wall: saturated clay is what pushes walls over.
More detail
Walls do not usually fail from weak blocks or thin concrete; they fail from water, and the drainage package is the difference between holding soil and holding a pond. The system has four parts that work together. Free-draining backfill: a column of clean gravel directly behind the wall gives water a fast path down instead of letting it stack up as pressure; backfilling with the native clay that came out of the cut is the classic homeowner-special mistake, because saturated Blackland clay is both heavy and expansive, a double load the wall was never sized for. Filter fabric: a geotextile between the gravel and the retained soil keeps fines from migrating in and clogging the gravel over the years, which is how good drainage quietly becomes no drainage. Collection and exit: a perforated pipe at the base of the gravel column collects the water and, critically, daylights somewhere downhill, because a drain with no exit is a French drain-shaped decoration; weep holes through the face add a visible backup path. Surface management: the grade above the wall slopes away, and roof or slope runoff is routed around the ends rather than over the face. On taller engineered walls, geogrid layers tie the wall into the retained mass, and the engineer's drainage detail is part of the sealed design. Every wall scope here itemizes this system by name.