There are two challenges you face when trying to build a bigger, stronger upper back:
In today's world, most people are plagued with iPhone posture. As a result, your pecs are in a chronically shorted position. This makes it difficult to get a full, peak contraction in your upper back. Overactive pecs also create reciprocal inhibition, which de-activates your upper back muscles.
Poor posture leaves your upper back muscles in a perma-stretch position. This combined with tight pecs and the fact that you can't see your upper back makes these muscles very hard to recruit.
Here's a simple, no-equipment-necessary solution:
Go slow and breathe out as you move into the stretch. Focus on flexing your upper back as hard as you can, then rest about 15 seconds and do your upper back work.
This will provide a nice dynamic stretch for your pecs. It'll also active your rhomboids, mid traps and lower traps. Best of all, this isn't something you need to add to your daily schedule or your warm-up. Just do 4-8 reps during your normal rest period before each set.