Thanks, chaps. Sorry I didn't reply sooner.
Helping others helps me, too. Just last week I stopped to help up a pensioner who fell over at a bus stop whilst a few others just kept walking. She was upset at having no money to pay me for my kindness but after I said, "Old lass, I don't need paying for doing the right thing," she got a tear in her eye and gave me a kiss on the cheek. Another bloke, who I vaguely know, also stopped and gave her a lift home in his van. I felt ten feet tall after that little act of kindness. Definitely powerful medicine.
I think getting out, taking the dog for a walk, especially with a friend, is also good for me.
One thing that might help others is how to deal with "dark thoughts." I used to get these a lot - you know the kind of thing, imagining punching someone or hurting yourself - and then worrying about them for ages after. I realised they're just random thoughts, everyone has them. Now I just shrug and say to myself, "it's just a thought, it doesn't mean anything," and let it go. I own my thoughts, or try to. Appreciate the "good" ones and cast the "bad" ones aside.