The rhomboids are key muscles in the mid/upper back that that are very important postural muscles.  They squeeze the shoulder blades together which supports the upper back.   They become weak due to the prolonged amount of time people spend in seated positions.  

If the rhomboids are weak you will notice that your shoulder blades will become further apart.

To begin with, the following basic rhomboid strengthening exercise should be performed approximately 3 times daily. As your strength improves, the exercise can be progressed by gradually increasing the repetitions, frequency or duration of the exercise provided it does not cause or increase pain.

 

Shoulder Blade Squeeze 

Begin this exercise standing or sitting with your back straight. Your chin should be tucked in slightly and your shoulders should be back slightly. Slowly tighten your rhomboids by squeezing your shoulder blades together as hard and far as possible provided the exercise is pain free.  Hold for 5 seconds and repeat 10 times.  

(http://www.physioadvisor.com.au/8292950/rhomboids-exercises-rhomboid-strengthening-exerc.htm)