For DS's first 9 months or so, I always had 2 outfits packed in his diaper bag when going out. (And I always took the diaper bag). Oh and it had a thin receiving blanket that could be used for multiple stuff. After that, it dropped down to 1 outfit unless we were going to be gone all day.
Every once in a while, I think about taking that outfit out -- then have an incident that makes me glad I did not. So he is 30 months and we are still carrying the (smaller) diaper bag with wipes, extra clothes, diapers, and all in it.
Married 3/04
DS 8/07
DD born 8/11