First Impressions

How do you determine what goes into the very first line of a short story or poem or novel? How you decide what you want the first impression to be? And how do some lines become famous almost to the point of cliche, while others, even on popular books or famous classics, remain comfortably in obscurity?

There are, of course, the cliches:
"It was a dark and stormy night."
"There was once upon a time..."

Then there opening lines which don't become cliche, but do end up so well-known that pretty much everyone knows where they came from:
"It was the best of times, it was the worst of times."
"It is a truth universally acknowledged that a single man in possession of a good fortune, must be in want of a wife."
"In the beginning, God created the heavens and the earth."
"Call me, 'Ishmael'."
"Marley was dead, to begin with."

And then there are works which have no real punch to their opening lines at all, yet the greater work still retains enough punch to render the book a classic:
"Christmas won't be Christmas without any presents."
"Here is Edward Bear, coming downstairs now, bump, bump, bump, on the back of his head."
"The first place that I can well remember was a large pleasant meadow with a pond of clear water in it."
"All happy families are alike; each unhappy family is unhappy in its own way."
"Unemployed, at last!"

So what goes into the thinking up of the very first line?

Thoughts?