I'm 35 and I've always known I don't want kids. How do I keep this up for the rest of my life?

We've all seen the women who were adamant on not having children only to change their minds when they turned 40. What is the key to not becoming one of those women, and to not become one of those women who is bitter and resentful about the choice not to have kids in the first place? How do you not let everyone else's opinions ruin your decision when it's one of the most personal ones you will ever make? My brother has tried to convince me to have kids lately.