Because my wife is taller than I, my shoulders almost always are set in a higher position than they would be if I were dancing with someone my height or shorter. So, it is easy for me to tell if there is too much tension in my shoulders -- they start to feel stiff.
Yoga is another good practice for relaxing parts of the body, and the shoulders being relaxed is a key to proper yoga as well.
I have enjoyed the tips and thoughts on this topic, as I consider it pretty important for a beginner to understand the difference between a proper frame with the head up and shoulders down vs. lifting everything up in an effort to "stand tall" -- yet look a little awkward.