A friend is one who walks in when the rest of the world walks out.-- Walter Winchell (1897-1972) US journalist, broadcaster

Don't allow the grass to grow on the path of friendship. --Native American Proverb

Go oft to the house of thy friend, for weeds choke the unused path. --Ralph Waldo Emerson

I can trust my friends. . . . These people force me to examine myself, encourage me to grow. --Cher

It is easy enough to be friendly to one's friends. But to befriend the one who regards himself as your enemy is the quintessence of true religion. The other is mere business. --Mahatma Gandhi

And my personal favorite:
Nobody sees a flower really; it is so small. We haven't time, and to see takes time -- like to have a friend takes time. --Georgia O'Keeffe