For those of you who liked the BBC's "Little Angels" child behaviour "expert" Tanya Byron, I have to say, her "Your Child Your Way" book has been really helpful in dealing with my own DS1's more challenging moments.
Basically the first part of the book encourages you to see the stage of development your child is at, psychologically, and it's helpful in getting you to re-adjust your own expectations of his/her behaviour. It is often a question of modifying your own mindset towards your child before you start trying to shape their behaviour more actively.
I found the book interesting to read, and useful to go back to if I am having a bad day. Lots of the behaviour management tips mentioned by other posters (e.g. distraction, praise, rewards etc.) are also covered in it if you want more practical help.
I've also found it true that if you can avoid putting yourself in a situation where time is of the essence, you put yourself and your child(ren) under less pressure, and are able to ride out the storm. Of course, that's not always possible, but if, for example we are headed out for a playgroup or the park and he decides that he wants to go to the loo and wash his hands ALL BY HIMSELF - then I try to take a breath and let him get on with it - and accept that we are going to be late (or not make it at all!)
But at the end of the day, I don't think there is one parent on this earth that has a child who never tantrums or misbehaves from time to time - it's just part of their development from needy little dependents into slightly larger, slightly less needy little dependents. You are still the centre of their world, and they love you to bits, but sometimes just lose it.
And anyone looking on as your kid throws himself down on the supermarket floor and starts screaming is just as likely to be sympathising with you as critiquing your parenting style, so don't feel you have to make any excuses to anyone other than yourself.