1. Length of line.
2. The "cashier"(The ones next to my house are either extremely slow, flirty or quick so I go wherever the quick one is.)
3. Number of items on people's carts.
4. Mood of the day.(No really-If I'm angry I always take the ones nearer the exit door without realizing it at first.)
5. Closest to where I am.
4. The cashier. If the cashier doesn't look very friendly or if I have a not-so-good interaction with him or her in the past I will try to avoid this cashier, even if its the shortest line. I remember one time a cashier made me uncomfortable because she'd say things like "You sure you want to eat that?"
I'll also be wary of getting into lines where I know the clerk looks like he or she will talk your head off. I just want to pay for my stuff and go. So in summary, I look for someone friendly but businesslike.
5. The people in front of my in line. I've noticed that the elderly tend to be slower emptying their carts and parents with children tend to be slower as well.
5w6 or 9w1 sp/so/sx, I think