One thing to consider - try to make sure all sizing and starch is out of the fabric for the block that will be signed. I completed a signature quilt last summer, and many of the signatures faded, some badly, after the first wash - which thankfully I did so that the recipient wasn't horrified the first time the quilt was washed. I had heat set every signature before washing, and used the appropriate pens, but I think it was the sizing and starch that did me in.
Ironing freezer paper onto the back of the blocks definitely made it easier for people to sign the blocks.
Cheers, K