Several studies have found that if you get people to write down things they're grateful for, they report better mood and more satisfaction with life. (This is compared to people who wrote down gripes, or people who did some other unrelated task.) One study assigned people to write and personally deliver a thank-you letter to someone who had been kind to them but had never been properly thanked. The letter writers reported increased happiness for a month afterward — and I imagine the recipients felt good, too! A gratitude journal, in which participants daily wrote down three good things and what the causes had been, had even greater effects. The journal writers experienced increased happiness and decreased depression for six months afterwards.
You don't even have to be some kind of naturally grateful person. Counting your blessings makes you happier even if you do it just because some researcher asked you to. It doesn't have to well up spontaneously — make it a habit. Part of the childhood bedtime routine I had with my parents was listing things we were thankful for, and now that Jeff and I are married we do the same before falling asleep.
And what does this have to do with giving? If you enjoy what you have, you're less likely to clutch at more. It's easier to let go.
It being Thanksgiving Day, I'll close with a table grace that's popular in my family:
Thank you for this food, this food
This glorious, glorious food!
And the animals
And the vegetables
And the minerals
That made it possible.