Something my friend Courtney Carver said has always stuck with me: NO means "no, right now." In the linked post she writes about saying NO so that you can say yes to what matters. There's also the other side to this. What if you are the one that is being told NO? What if it's your work that's being rejected?
It's tough but I try to remember these things. It doesn't mean the person doesn't like me. It's my work that's being rejected, not me. It's NO, right now. It's not a NO to future work or this work at a another time. It may be NO because it's not in the budget or it doesn't fit the particular project. It's not necessarily NO because your work sucks! It's NO, right now. That's all. Move on. Keep moving.
Another take on this same topic can be found here via Carli of Inkedfingers.