How did Evolution produce Symmetry?

It is an interesting fact that out of the vast variety of multicelled creatures, most are entirely symmetrical - especially the vertebrates. How could a random process of evolution produce animals that are completely and perfectly symmetrical, instead of having two heads with three eyes on one and none on the other... or invertebrates with an uneven amount of legs? Most creatures adhere to the laws of symmetry.
Could anyone here provide a hypothesis as to how evolution would produce such perfectly crafted creatures?

Further thought will be required by all. I've never heard this topic actually broached before. But it seems clear that, while some symmetry is good for survival, much is not. An extra eye on one side of our head would be asymmetrical and probably beneficial as well. Why is it then that if you divide the shape of a human body in half you get bilateral symmetry? Ten fingers (2*5), two arms (2*1), two eyes, ears, and a nose precisely in the middle. Sounds simple but it's actually quite perplexing from an evolutionary standpoint. For example, if eyes evolved from light-sensitive cells, why didn't these cells pop up all over the place? What we have is two perfectly aligned eyes pointing forwards.
And of course there are the decorations, such as completely symmetrical designs on butterfly wings. How did evolution come up with that?

Yes, I did sound a little patronising up there. As [Mr Jack?] assumed, I was emphasising symmetry. Five fingers on each side, not four and six or three and seven. Everyone knows that! It's good to restate it sometimes for the purpose of bolstering your argument.Just a quick note before I sign off for the night - I noticed something schrafinator said a while back: What about the artists responsible for, say, the Easter Island carvings? We cannot see, touch or hear them, and (as far as I know) we have no evidence for their existence apart from their art. Yet we still infer their work is the product of intelligent design.

Does this mean, Zephyr, that if we had no evidence concerning the history of the moai, we could assume they are not the product of intelligent design?

Let's try a hypothetical situation. Say you're diving at sea and you discover a carved marble statue. Would you say it was intelligently designed?