This very issue is why I call the support group I run "Strength Through Laughter" -- because it's so important to find those moments when you can laughor feel positive, even if you're laughing through your tears. The most important thing is to never stop hoping. It doesn't help to be dishonest and discount what's happening; when your parent's in pain and suffering, you want to acknowledge that. But you also want to help your mother look for hope whenever she can.
A patient of mine said something the other day that was so positive and helpful that I wrote it down to use again later. This woman, who's very old and sick, said, "They say there's a one-in-300 chance that a person with my diagnosis will get better, but how do I know I'm not that one person?" You just have to think that way when you're going through treatment, because what else are you going to do? Encourage your mother to think of herself as number one and to think about things that make her happy -- even small things, like a vase of flowers on the table.
Another important strategy for helping your mother keep a positive attitude is to empower her to stay away from people who make her feel sad or discouraged. One of our slogans in our group is to stay away from negatives. We laugh about it and say, "Screw anyone who brings us down!" Sometimes people think they're expressing sympathy by telling a cancer patient about their own or another person's illness or sad story, but that's really not helpful at all. Encourage your mom to feel comfortable saying that she doesn't want to hear about other people's sad moments right now. It's okay for either of you to tell people that she needs her strength to go through treatment, and she needs to be protected from things that bring her down.
Finally, celebrate everything -- birthdays, anniversaries, the end of chemo. Nothing's too small to celebrate. None of us ever knows how much time we have left, and this is a great way to make the most of what we have.