What is an appropriate tip? (Open Thread)

When dining at a restaurant tipping is pretty easy; common practice is usually 15-20% of the bill. But tipping delivery service employees is different because there are many factors involved beyond the size of your order. First and foremost is the distance you are from the store, sure you may have only gotten one pizza but if you live all the way across town you're costing the driver more of their time. Another is the weather, if its pouring rain or snowing heavily it seems logical that the delivery person should be compensated for going out in that weather so you don't have to. Then there is time of day; is it rush hour and is it more than likely that the driver had to sit in traffic on the way over to your place? or is it five minutes before the store closes and you know that they'd rather just not take your order at this point, but do anyway? Last but not least is of course how long it takes them to get there; which obviously plays a big part in what most people decide to tip. Being a delivery guy I always think about these factors but does everyone? Do you think about any of these when you decide how much to tip the guy hand delivering food to your door or is it just a spur of the moment decision?