Ernestine Schumann-Heink (1861-1936) was the principal contralto of her time. In 1921 she had this to say on practicing:

“I know nothing better for acquiring a flexible tone than to sing trills like the following:

and at the same time preserve a gentle, smiling expression. Smile naturally, as though you were genuinely amused at something  – smile until your upper teeth are uncovered. Then, try these exercises with the vowel “ah”.

Don’t be afraid of getting a trivial, colorless tone. It is easy enough to make the tone sombre by willing it so, when the occasion demands. You will be amazed what this smiling, genial, amiable expression will do to relieve the stiffness and help you in placing your voice right. The old Italians knew about it and advocated it strongly. There is nothing like it to keep the voice youthful, fresh and in the prime of condition.”