How to successfully kill off a character

ok when I was writing most of my books I noticed that it was very hard to kill off characters. Not just the fact that you will have to write the other characters responses, but the fact that most authors( like me) usually get emotionally attached to their characters and usually feel really bad when they have to kill off a character( expecially a character which is good).
When I was writing TC, I knew that I had to kill off a main character. Now it is not mandatory that you kill of a major character, and frankle I don't recommend it. Back on topic about Tc, I had spent a long time thinking and rethinking Trevor's death. How would the other characters react? Good, bad, or a mixture of feelings?
Writing death is the hardest thing for a writer to do( other than action sequences, which I will explain later). I managed to make a couple tips for writing death:
1. Pick your characters right. Think: if I kill off the main charater how will the audience react. Will they stop reading the book because the most liked character is dead, or will they be eager to read on.
2. Do not kill of the youngest character(characters that are younger 18). The audience will think your morbid
3. Don't go into too big description of their death. Try to make it as censored as often, but don't just say "Suddenly he was killed". That bores the audience and takes them too much by suprise. Make it descriptive but not boring(expecially not gory!), but not too less descriptive that it doesn't make sense
4. Will your character's death be a sad event or a happy event( if it is happy it better be a villian's death!). How will the others characters react?
5. Dead is dead, and it sounds odd to randomly bring a character back to death. It makes the reader think that you can't write death. Any author can write death, but rarely any can keep that character dead
6. Should a character avenge the person. Now this a commonly used method of starting an adventure, and it is easy to write. There are two ways to use the whole avenge thingy: a) They go on the adventure ONLY to avenge the person's death. b) use's the person's death as an excuse to want to kill the murderer

Now most of those tips I broke when I was writing TC. Though that was the first book where I had killed off a major character. Well I've tried to limit down on my character deaths. Most of the time I give them near-death experience. It is easy, it lets you write a death sequence, but still keep the character. Well I'm going to keep writing, and I'll keep posting later.
-AM