
common sense stuff :: basically, what i mentioned above. be communicative and approachable. let the person know that they can talk to you about their problems and that you'll listen, give them support and possible solutions and that you won't judge them for their feelings. encourage them to talk to you when they're feeling down rather than simply dwelling or feeling worse. try to stay positive for them, without glossing over what they're feeling and encourage them to see positive solutions or progressions from the pit in which they find themselves. people with mental disorders often resist treatment out of fear or suspicion of strangers, so help by offering to accompany them to therapy, or meet up with them afterward to discuss the experience- anything to make the process of getting help more comfortable. helping them taking action will also, ultimately, be empowering for them.
know yourself :: lots of people have prejudices about mental disorders and, if someone close to you has been diagnosed with or shows signs of one, it's important to be honest with yourself about what your thoughts are. in your heart of hearts, do you think depression is a sign of weakness? do you feel that medication for mental disorders is always a bad idea? do you even believe that mental disorders are real illnesses? your own prejudices limit how much you can help someone else and a lot of people find it very difficult to set those prejudices aside. i don't really advocate telling someone if you don't really "believe" in their disorder for what i assume are obvious reasons. what i would advocate is listening. hear what they have to say and try to keep your opinions as objective as possible. if it's making you uncomfortable, try to encourage them to talk to people [such as professionals] who might be better positioned to help.
educate yourself :: we live in the age of readily available information. within half an hour, you can learn more about the functioning of the human brain than most medical doctors knew a century ago. be discriminating about the reliability of various sources, but find out as much as you can. if your loved one is on medication, read up on them. what are its side effects? what alternatives are available? how does it work? how soon should they see a difference? what happens if they take too much or stop taking it abruptly? find out what doctors and laypersons have said about their condition. is it permanent or does it go away? what are the outward signs of it getting better or worse? how have other people successfully dealt with the same disorder?

perspective is important :: this goes both ways. if you tell someone that their mental illness is no big thing, you're making them feel like a loser for not being able to deal with it. if you make it seem like an unconquerable monster, you'll discourage them from thinking they can do anything to help themselves. again, positivity helps. problems happen, but they can be dealt with.
remember, they're crazy :: that might sound a bit harsh, but, when you're dealing with serious problems like bipolar disorder, schizophrenia or severe depression, you have to keep in mind that the person you're talking to is not necessarily reliable as an information source. people with mental disorders can be extraordinarily manipulative and can often trap you into saying things that feed into their depression, paranoia or other ideation. they can also be very resistant to social contact and can go to great lengths to distance themselves from other people. learn to recognise the warning symptoms [part of educating yourself] of a "bad turn" and keep in mind that, at such times, you may need to take whatever they say with a grain of salt.
their crazy is only part of them :: don't assume that everything they do, say or think is a result of their mental illness. there's a lot more to people than that. when people are in the throes of a mental crisis- a depression, mania or "dissociative episode" [reality bye-bye]- people can say things and do things that they would not otherwise do, but they aren't always in that state. much of the time, they're likely to be on a fairly even keel and you can bet that when someone gets angry at you because you said you'd help them move and then showed up drunk, or because you were supposed to water their plants while they were on vacation and forgot, or because of any other reasons that would annoy anyone, you can't brush it off by saying they're crazy. that's normal. and if you keep ascribing every aspect of their behaviour to their illness, you're only going to make them crazier.
ultimately, there is no one correct procedure for dealing with someone who is mentally ill. the good news is that there are a lot of ways. i think a lot of these are pretty straightforward, but bear repeating. one that i don't think gets nearly enough importance placed on it is education. there is a lot you can learn about mental health and you might be surprised how out of date popular knowledge is on the subject. and the only thing better than having someone close to talk to is having someone close to talk to who understands what you're going through.
you may have noticed this week that the images i've selected are not quite as random as usual. that's because they all come from artist joey chou and his book "crazy by the letters", a sort of "gashlycrumb tinies" for the mental health set. buy his book here and explore the rest of his site to look at more of his art.