How to get the previous element when using a for loop?

Possible Duplicates:
Python - Previous and next values inside a loop
python for loop, how to find next value(object)?

I've got a list that contains a lot of elements, I iterate over the list using a for loop. For example:

li = [2, 31, 321, 41, 3423, 4, 234, 24, 32, 42, 3, 24, 31, 123]

for i in li:
    print(i)

But I want to get the element before i. How can I achieve that?