hey man, if you are like any of us, and spend a fair bit of time in front of a computer, ur upper back and inner shoulder-blade muscles are not too good to start with.

i started getting really sore the first few 2+ hr rides out with my buds, but i found 2 things helped a lot:

1. focus on relaxing ur shoulders and pushing out ur chest when ur on the bars
2. bend elbows.

i ran with drops and now with pursuit bars, so I don't know how well this applies to flat bars.. shorter stem may help as well.
but, correcting posture is far better than swapping parts and still riding with messed up posture.