Helen, I think it's awesome that you are even asking this question!
My mom doesn't necessarily understand my choice (she absolutely couldn't wait to be a mom, had 4 children and would have had more if she could've talked my father into it!), but she really does try. One of the most supportive things she's ever said to me is "I can understand not having children . . . they can be alot of joy, but they can also be alot of pain". I appreciated that as so often we hear "you're going to be sorry", "but you are going to miss so many things", etc. Not very often do parents acknowledge the challenges and heartbreaks that some children can bring. My mom has one close childfree friend and several times has commented on what a nice life she has had . . . lots of travel, freedom, retired early, etc. and that the childfree couples she has known seemed to have good marriages-it's nice to hear her make the effort to recognize the benefits of my choice, especially when it's contrary to her own. My mom also includes my four furry kids in any correspondence she sends . . . it also makes me laugh to see their names on the envelope!
OTOH, she just recently pointed out a couple of small children to me in a restaurant and said "Aren't they cute? Wouldn't you and DH like to have a couple of those?" I'm like, Mom, I'm almost 40 and DH and I have been married almost 15 years . . . if we wanted kids, don't you think we'd have had them by now??? I have 3 brothers who have kids (11 total, so she is a grandma many times over), but my mom has also commented that it's different when your daughter has a baby. So, those would be examples of what not to say!
The most meaningful thing that my mom could say to me is "I'm proud of the fact that you have obviously done alot of soul searching, made a decision for yourself that is contrary to what society expects of you and that you are walking your own path, not one that you let other people define for you. I admire your insight and courage". That would mean the world to me!