Don't try to "fix" the problem, because you can't.
While everyone else is saying, "Things'll get better!" because they're supposed to say things like that, the friend that acknowledges, "Dude, that fucking sucks" is a friend indeed. Imagine, you feel the Feds are out to get you, but since you’ve been diagnosed schizophrenic, you tell yourself they probably aren't. You see cars coming at you even though you're outside your apartment and nowhere near the freeway, so you will yourself to keep walking. But you know with certainty that this situation sucks, and there is no cure - but then everyone around you tells you that everything's actually going to be ok! Wouldn't that just make you crazy?
It's painful to see someone you love suffering, and you feel compelled to do something, but this is not the time for platitudes and cheery kitten cards. Things that cheer up most people don’t work for someone in the valley of clinical depression. Remember that mental illness is an issue of the brain muscle, not willpower. Resist the urge to barrage your loved one with suggestions to solve the problem. Chances are, he or she has already thought through many of your suggestions, as they're merely sick, not stupid, so let the doctors do their job.
Do educate yourself
People who are fortunate enough to get their mental illnesses diagnosed have one of two stories to tell about their loved ones’ reactions: either they felt abandoned (by a parent, for example) who refused to acknowledge or learn about the illness, OR they felt loved when a beloved went out and read every book on their diagnosis. The most genuine gesture of support you can give is to educate yourself about your love's diagnosis.
- Good Old-Fashioned Books are a good start. I've listed some of my personal favorites in the sidebar, where you can buy them from Amazon directly and support this site.
- National Alliance on Mental Illness is a nonprofit with excellent resources.
- National Institute of Mental Health is part of the government supported National Institutes of Health research organizations.
- The Icarus Project is a nonprofit network of people living with mental illnesses. Their site also has great free information.