I found with DS that shirts that have 'top down' nursing access instead of 'bottom up', promote better posture.
Right now, I'm actually nursing and typing with 2 hands :)
Cate's head is tucked on my left forearm near the elbow, nursingon the left side. I'm sitting on the left side of my rocker (it's a wide one, I'll share a photo of it later, it's not like a traditional one at all), with my elbow propped on the arm. Her tushie is more or less on my right leg, close to my body, and her feet are hanging off the side of my lap. My feet are on my little angled nursing foot stool, and the coomputer is on a TV tray/table pulled up close over my knees. I have one big pillow behind my back, b/c this chair is so deep.
When I sit like this, I am nearly sitting up straight, and if I stop typing, I can be sitting up totally straight :) Cate's over all position is like a semi-reclined sitting position with only her head turned in towards me to nurse.
That saves my back/neck/shoulders.
If you are using shirts you pull up to nurse with, try wearing some deep V-neck shirts over your nursing tanks/bras (or try the Undercover Mama 'shirts'), and see if pulling a neck opening down helps. W/o changing anything else last time (pillows, support etc.) I found just the wardrobe adjustment to help with my nursing posture.