Here is some good advice I myself received from books, conversations, examples I found somewhere, I just pass it on to you because most of it I have tried out myself with really good results, but please don't blame me if something should go wrong for whatever reason.
So: If there are no stains on the quilt, it is usually not necessary to wash it. Washing puts a great strain on the fabrics and the yarn, and in most cases it is perfectly ok to just air it. Give it as much fresh air as you can, I hang my bedquilt over a chair at the open window each morning for at least 15 minutes. Stored quilts : maybe there is a possibility to hang them over a line outdoors in summer, too.
As a general rule: avoid direct sunlight on the top of the quilts and keep them at moderate humidity and temperature. Avoid unnecessary folding. If you have the possibility, lay the quilts you don't use flat on a spare bed, with pieces of cotton or linen in between. If you can't do this: roll the quilt around a cardboard tube and cover the tube with a piece of cotton, linen or tissue paper first, then cover the quilt with cotton.
If you have to fold a quilt : rearrange and refold the quilt from time to time then. If you keep quilts in a chest: avoid direct contact with wood.
Never keep a quilt in a plastic bag, plastic prevents fabric from breathing and it can trap moisture then.
A good idea is to give a cotton-bag, filled with Lavender, to the stored quilts: it provides a wonderful aroma and it keeps moths away.
Some people say it's a good idea to carefully vacuum a quilt, but I never did that.
In case you feel it's absolutely necessary to wash your quilt:
Before you wash your quilt for the first time, put it into a large tub which is filled with clear cold water for at least 8 hours. This will prevent any shrinking of the cotton parts of your quilt, be it fabric or yarn.
After that:
Before you wash your quilt for the first time, put it into a large tub which is filled with clear cold water for at least 8 hours. This will prevent any shrinking of the cotton parts of your quilt, be it fabric or yarn.
After that:
Use shampoo, cool water, wash it in a large (bath)tub, rinse carefully, wrap it in cloth to lift the now very heavy quilt out of the water (so that the threads don't rip), wring carefully and lay out flat on towels (maybe on the lawn, in the shadow, on a warm summer day).
In my experience, however, this is a job for two persons because the wet quilt gets verv heavy and the whole process requires a lot of space and good weather.
So I decided to use my washing machine with a delicate quilt, manufactured with lots of cotton fabric and cotton yarn, no pre-washed parts, and it all went very well.
I put the quilt into the washing machine, let it get wet with COLD water using a wool program but no detergent at all (it took 20 minutes with my machine until the quilt was really completely soaked and I stopped the program there), let it sit for 8 hours, then waited until the water was pumped out.
After that I used the wool program again with low temperature (30°) and a special detergent for wool, and let it spin at the lowest possible speed of my machine (600). After that the quilt felt very good, it was a gentle washing cycle and the quilt wasn't so heavy, which reduces the strain put onto the seams and it can be hung up easily. I used a laundry rack for hanging, that distributes the weight of the quilt best. While the quilt got dry I rearranged it several times and kept it in shape.
The next time I need to wash this quilt I will only use the wool program again but not the 8-hours-soaking-in-cold-water-time beforehand, this is only necessary when a quilt is washed for the first time.
