If you pull up to the gas station and the truck is there that fills the gas tanks, don't buy gas at that time if you can avoid it. When the tanks are being filled by the delivery truck, it agitates the gas in the storage tanks which in turn stirs up the sediment in the tanks. If you pump gas at this time, you'll get sediment in the gas that you are buying. It's not horrible for your engine since all cars have filters that take sediment out of the gas if there is any, but it will reduce the life of your fuel filter, so you are better off waiting if you can.