How can I help someone with mental illness improve his or her self-esteem?

See Below. Often times we focus on the negative aspects of self or environment. We have to lean how to refocus attention from negative onto positive. For example, you might want to ask the person to find 3-4 positive things about self and right them down, you can also try to do a positive summary of the day -- best food i ate, best jokes i left, things achieved, etc. It's a constant work, but worth the try.