[enan]

Here's the trick I've successfully used during peak hours. All things being equal, pick the longest line next to a closed lane, wait for the closed one to open (supermarkets will usually open additional lines when they think they are getting longer), and then quickly switch.

[colanderman]

It's expected that you let the person who's been waiting the longest move there. That's why you'll hear the new cashier say "I can help the next person in line." I consider hopping over from the back of a line to be very rude.

[joezydeco]

I've also seen clueless managers at some stores (cough cough TARGET) take the people in the back of the lines and redirect them to the newly opening lanes. Pisses me off to no end when I'm the guy that's been waiting the longest and grandma in front of me pulls out her checkbook....

[enan]

Of course, but many people will never switch. And even if they do, you will be in a line that is shorter than others.