I personally know a handful of women that are so unorganized that even keeping a packed diaper/nappy bag is difficult for them. So I thought I'd share what I do.
I have two diaper/nappy bags: one is a small backpack that has everything I need for the Bubba if we're going out for a few hours. The other bag is a larger bag for if we're going away for the whole day or longer.
In the smaller bag I keep:
5-6 disposable nappies/diapers
travel baby wipe container
change of clothes for the Bubba
2 bibs
2 baby bottles with cooled boiled water, ready for a drink
measured formula in a container
ziploc bag with snacks for the Bubba
a toy to keep the Bubba occupied if she becomes cranky
sunscreen
my camera (always carry my camera with me)
In the larger bag, as well as the above items, I keep:
more nappies/diapers, if we're going to be away longer
a bunny rug
small makeup purse for myself with headache tablets, lipgloss, bandaids, deodorant and breath freshener
When I get home, I repack the nappy/diaper bags as soon as I walk in the door so I know that the next time I need to run out the door, the bag is packed and usually all I need to do is fix up some bottles for the baby.