Heres one that works well...
With your client supine, reach under the clients torso to the origin of the rhomboids with your fingertips, and then laterally extend the clients arm (like you are stretching pecs major).
the weight of the clients body will provide good pressure. Slowly strip inferiolaterally along the rhomboids until you reach the medial border of the scapula. continue that pressure pulling inferiolaterally on the scapula, while laterally flexing the clients arm across his body, which will cause the scapula to move away from the midline, and further lengthen the rhomboids.
This has worked very well with some of my clients (pro athletes).
If you try it, please let me know how it works. I have a few more effective active techniques if you are interested.