A tip if you're making rice/wheat bags: make sure you don't use a strong coloured fabric. I bought one once which had a purple fabric, and it stained all my sheets blue. A friend bought me a shaped rice bag for my shoulders the other year, and I've never dared use it because it's red.
It can be a good idea to make an inner pouch, which will be filled with rice/wheat and sealed, and an outer removable one which can be washed. Unfortunately, this occurred to me after making a couple of very pretty one-piece embroidered rice bags!
One of the most important things for me for preventing pain when quilting is to take lots of breaks and do stretches. I strongly recommend stretchclock.com for this purpose, it's free and you can set it to remind you to do the little stretches as often as you like. They're designed for office workers, so they target the right spots: back, shoulders, hands etc.