Mod Warning
This thread contains spoilers about the ending of Mass Effect 3. Continue reading at your own peril....
I really think people are focusing too much on getting a 'perfect' ending. It's a role playing game! Play Shephard as you would like your character to be not always worrying about what cut scene you're going to have at the end. The beauty of this game ultimately has been that you will have an experience all the way through that reflects your personality as you played (whether what was really yours or one you decided to be). Don't get pissed off that you made X decision in ME1/2 that led to Y dying, or not helping you etc. just live with it and move on knowing that the universe you are in now actually does a great job of reflecting everything you have done so far. It's yours

, I don't know of any other RPG that has taken this aspect so far, the fact that it's stressing so many people out is testament to this. Basically you can't get it wrong, you can have different outcomes but they will reflect your in game persona from all 3 amazingly well.