When I was very young, a good friend of my mom's killed himself. I don't remember how. His name was Mark; my sisters and I called him Mark Dark, because he was into horror movies and liked to scare us with silly monster masks. Really sweet guy; if I remember correctly, he killed himself because he was having problems with his girlfriend. This was around '79 or '80, and he was my mom's age which means he would have been just shy of 30.
My best friend hanged himself when we were in basic training together. Our head DS announced it during morning formation. He never said a word to me before or showed any indication that he was struggling. It was a complete shock. I remember breaking down right there in formation and one of the reserve drill sergeants taking me aside to console me.
I had another very good friend who drank himself to death. It wasn't technically suicide, though; he was an alcoholic who happened to go on a bender after having an argument with his wife, and he overdid it. He died sitting on the end of his bed; coronary heart attack. His BAC was .50. Again, not technically suicide, but he knew he was an alcoholic and knew that when you're an alcoholic, one drink is too many and a thousand is never enough. I think he had to know when he opened that first bottle that he wasn't going to be coming back.