This take is in response to a take by @AngelicSin Things You Should Never Say to Someone With Mental Illness Please go read her's first and show her support for her myTake.
A lot of the things people say to try and "help" friends struggling with mental illness are just... your friends have heard them before. They didn't make things better before and they won't now. I've had several friends go through these sorts of things and the following are those things I've found to genuinely be positive and encouraging to them.
1. "I'm Always Willing to Talk"
You can also say something like "let me know if you ever need to talk." I tend to say this, but I think "I'm always willing to talk" can be better in some regards. Saying "let me know" has the potential to put pressure on them. It puts the responsibility of reaching out on them instead of you. It's really hard for people struggling with mental illness to open up most of the time so placing the responsibility of reaching out on them can be bad thing. On the other hand one could argue that saying "let me know" gives them a bit more control which can be a comforting thing so which you choose to say just kind of depends on who your friend is.
2. "I'm not going anywhere."
Most people who have struggled with depression, anxiety, or some other form of mental illness have had people leave because of it. We don't like to deal with it. It makes us uncomfortable and so we run away. That makes people who have these issues paranoid in a certain regard. (Don't get me wrong, calling it paranoia doesn't mean I think their concerns or worries are at all unfounded). One of the best and most encouraging things you can do for someone is to stick around. When it gets hard please stay beside them. It's one of the hardest but most beneficial things you can do for them. Letting them know that you're willing to stay beside them even when the going gets tough... that can mean the world to them.
3. "I love you" (please read this one)
Please be very careful about using this one. It has the potential to be a very positive thing or a very negative thing. On the one hand it can be one of, if not the best way of saying, "I care" because at this point... "I care" is a phrase that has lost a lot of meaning. Enough people say it just to feel better about themselves that, while it can be very helpful or meaningful to a person, it's also easy for them to write it off as your "just saying it." I love you is still a powerful phrase that has the potential to show that you're genuinely invested in and care about a person. However, it can also lead to bad stuff. It's easy to fall in love with someone who says they love you. That's where the problems arise. If you say I love you to someone with mental illness you must make it VERY clear exactly what you mean when you say it and you must do whatever it takes to make sure you're not setting yourself up to be the person they wholly rely on. You aren't the cure even if you can be part of the treatment so do whatever it takes to avoid leading someone on. Never let someone feel like you're the answer to all their problems because if nothing else, should anything happen to you or if you have to leave at any point it will destroy them.
4. Compliments in General
I once ended up in a video call at 11 p.m. with a close friend of mine. They happened to have a panic attack while we were talking. One of the things that stood out to me was how much my friend thought of themselves as worthless. It's what the voices in their head kept telling them and they'd begun to believe it. Giving compliments to someone be it about their personality, looks, skills etc, highlighting things that make a person unique and special can give them something to hold on to when the voices in their head tell them they're worthless. For the record, giving compliments is almost always a better form of encouragement than saying "It'll get better," or something like that. When it comes right down to it, you don't actually know if things will get better for them or not. That's why I think giving someone something to hold onto in the here and now is better than giving them something unknown that they can't see yet and may or may not ever see.
5. "Hey, are you doing okay?"
Make an effort to be the one who reaches out. Do it out of the blue. Don't wait til your friend posts something on facebook or twitter about how no one cares. That just makes you look like you're doing it out of pity or because you felt guilt tripped into reaching out. That makes people feel like a drag and that's the exact opposite of what your goal should be. Make an effort to be the one who reaches out. Check in on people out of the blue and make sure they're doing okay. If they're not be willing to talk, listen, or whatever else they may need. I like "Hey, are you doing okay?" over "how's it going?" or "what's up?" It gets straight to the point and it isn't a phrase to which people are super used to responding with "I'm okay," "I'm fine," or "not much." Please don't ask vague questions that people can escape easily. It may feel like you're pushing them which can be very uncomfortable, but believe it or not it shows that you really do care. ALSO! Realize if they simply say "fine," in response, you'll have to push deeper because "fine" only means fine once in about every 20 times.
Bonus: "My Pleasure."
Now I know people joke about being a good chick-fil-a employee when you say my pleasure a lot (or maybe that's just in my social circles), but this can be a much better response than "don't mention it," "don't worry about it," or "no problem." When you say "My pleasure," it communicates to a person that, not only are they not a drag or a hassle to you, but that you genuinely want them to reach out to you if they need you. People with mental illness often end up being some of the sweetest and most self-less people you'll meet and one of the things they worry about a lot is being a burden to others. If you can avoid making them feel like a burden, you've done a great thing for them.
I'm not trying to say that everyone should sacrifice their personal time to start some sort of mental health ministry. This take is for people who have friends that deal with mental health issues and want to help them. It's not easy doing any of these things. Sitting with my friend through their anxiety attack sucked. It hurts to see people in that much pain dealing with stuff you struggle to even imagine. But I want to encourage anyone who would seek to invest in a friend like this... it's 100% worth it. If you find you don't struggle a lot with your own problems, I'd encourage you to go out of your way to support others in theirs. Again, I don't want to demonize anyone who isn't doing this, I simply want to encourage those who are.
These are points are all based on my experiences with my friends who have struggled with mental health issues. Please realize that not all people are the same. Some need space and need to feel in control. Do what you can to learn about the people with whom you interact. If you meet someone that likes to be in control you can let them know that you're open to talk and that they should reach out to you if they ever feel the need. As I mentioned before that can make people feel secure and in control even if it's just in something small, and sometimes that's all a person needs. In the end mental health issues vary from person to person and so should your response. There's not a one size fits all system here. I hope however that you can use the points written here to gain a better perspective and a better idea of what you can do to help the people close to you.
If anyone reading this struggles with mental health please feel free to say what things you think would be more helpful or to clarify if one of these things WOULDN'T be helpful for you and why. I don't want to pretend I know everything and I don't want to lead anyone astray. So please be my fact checkers here in regards to your own personal understandings and experiences with the issue.