Recommendations for targeting Rhomboids?

Does anyone have any suggestions as to how I can really focus on targeting my Rhomboids? With regards to working out my back, my exercises include: Bent over barbell rows, overhand pullups, underhand pullups, seated cable rows. I have tried throwing in reverse cable flies with my elbows bent, but I find that my posterior delts always fatigue first (note: I've been a competitive swimmer/waterpolo player for 20 years, so I don't think its due to underdevelopment of my shoulders). Any advice would help--thanks in advance!