Sometimes, patients simply don't get it, so a modified position is required. I try four point kneeling or sidelying or on a fitball or a pilates reformer. The main idea, as GC said is to eliminate accessory movements. Keeping the pelvis neutral is really theoretical and not very practical at times especially if the patient has lordotic features and the patient needs to be able to contract the TA in standing. Ideally you can 'isolate' the TA with the pelvis neutral and perhaps lumbar flexion can achieve that with a lordotic patient. However, 'isolating' the TA is only the beginning of stability training.

Like with all stabilising muscles as 'gc' said, the patient eventually needs to be taken throughout range, flexion, extension, rotation, with varying loads placed on that muscle, such as with difficult postures, weight training and slow to brisk walking.