1 - Honestly it's all so subjective. I think the one thing I was missing a lot growing up was people simply asking if I was alright. Provide a shoulder to vent and/or cry on. Even something as simple as saying "if you want to hang out/talk about it, I'm here", or even "if you don't want to talk about it, but just get away for a bit and hang out", that would have helped a bit.

2 - Don't make someone feel like their problems are their fault, or that you're clued up as to why they feel the way they do. I never really had help from my parents, rather lectures on why I felt the way I do, based on what they perceived to be deep rooted psychological issues. I'm not saying you would do those things, but honestly, I don't think my parents intended it. Personally, if people who were close to me would have made it clear that I could talk to them, or offer to take me to see someone and things like that - that would have really helped. I felt like my parents really demonized the things I did, and not in a 'we're shocked and don't know how to react' way. More in a, "how dare you, we didn't raise you like this, you're awful", which doesn't really help.

It's all really subjective though.