1) "Good luck"
By the time your child is 3 or 4 years of age, if you've been using this phrase and philosophy from the first birthday, he/she will know that they are getting ready to mess up, or maybe mess up, or take a harder way than necessary, or be uncomfortable. It is a method of 'detachment' as a parent that as an observer of your child you are commenting on something he or she is getting ready to do. As long as the consequence or cost of the poor choice they are making is not illegal, immoral, or life threatening, 'good luck' is a very effective couple of words. You can start anytime, but the earlier the better.
2) "I'm sorry"
You (or at least I) have to watch the tendency toward sarcasm when using this powerful phrase. When your child is hungry, cold, hot, when he has failed a test or been allowed to forget to turn in a project and gets a '0', these words come in very handy. The mental mother in your head is standing there shaking her finger saying "if you had just done what I said, this wouldn't have happened". But don't succumb to her influence. It is far more effective to empathize sincerely and allow the child to come up with the "if I had listened to...". By the time the choices have illegal, immoral, or life threatening consequences, your 'good luck' should bring to mind many 'I'm sorry's' and perhaps a better decision.
3) "Nice try"
Instead of crushing your child's spirit when he throws a fit or makes an argument you can give him credit for the effort and keep the part of him that was passionate about and willing to sacrifice time and effort to get what he wanted. Jesus has a couple of parables with a 'nice try' in them: the parable of the wicked (or shrewd) servant where the guy is getting fired and he knows it and he makes deals with businessmen who owe his boss money so maybe he can get a job when he gets his pink slip. the other is the persistent widow who just pretty much bugs a judge who finally makes a ruling in her favor to get her to quit pestering him. In these cases, the effort is rewarded, and in the case of the widow, the 'nice try' even works. This can be accomplished with your kids in several ways. I've always thought that parents should have a set of flash cards with numbers on them, like 7.5, 9.0, 9,5, etc., to give kids a visual 'score' for their 'effort', whether it's a tantrum or just an argument. Just the acknowledgment that you noticed and appreciated their effort will go far toward changing behavior while maintaining 'spirit' and 'spunk'. I'm not recommending tolerating bad behavior, but I see and hear parents squelch their child's 'fire' when a 'nice try' would have been appropriate. Such a tactic can be hard in the day to day activity of dealing with kids, but it is worth the effort.
This, along with #5, is a "stopper". Use this when you've exhausted all the other tricks, when a 'good luck' won't work. When you have to say 'no'. After you've said 'no', and the argument/tantrum is going, you could give them a 'nice try' but still you have to stay 'no'. That's when 'regardless' comes in handy. "Regardless of the fact that all your friends' parents have bought them cell phones, we are not buying you one."
Another "stopper", this word takes into account all the arguments, acknowledges the effort put into changing your mind, and comes close to agreeing that your position is hard to explain or defend. It is maybe a longer and wittier version of "because I said so"."Nevertheless, you are in the 2nd grade and you may not get a tattoo."
Most parenting scenarios can be handled using these words or variations on this theme. Practice them when your kids are young, and when they are a little older, they will give up and beg you not to say 'nevertheless'!