Scapular wall slides are a potentially great exercise for improved shoulder rotation, upper back activation, scapular mobility, and anterior muscle release as a by-product.
To do them, stand with the heels, butt, upper back, shoulders, and full arms and hands against the wall, reduce the lower back arch, and slide the hands up and down, mimicking a full shoulder press movement pattern.
The problem is that you can adapt quickly to an unloaded mobility drill, and because of this, the wall slide can become a non-transferrable "skill" that doesn't carry over to improved posture or performance.
An Even Better Way
To remedy this, adding some mild resistance can "remind" the muscles of the rotator cuff to center the humeral head in the socket and create a much more effective external rotation position.
Resisted Scapular Slide
Using a neutral grip via ropes (as compared to a palms-forward grip) creates a more ideal and shoulder-friendly environment for external rotation that can act to counter anterior shoulder glide and ultimately put a real dent in bad posture.
For resisted scapular slides, use a cable pulley and perform the lift from a seated position. This allows you to focus on avoiding back hyperextension, which is a common compensation pattern when you have bad shoulder mobility.
This movement creates a force angle that works against the standard slide pattern, so keeping the hands and arms moving along the same plane becomes a more challenging task for the scapular muscles.