Not sure this qualifies as a hack - it's probably just obvious advice.
A few months back, I tossed a Ziploc bag with a couple of clean diapers in the trunk of the car. Tonight, due to an odd confluence of events (running w/ the little guy to meet Shira at the gym, a spilled water bottle, our interest in eating dinner out), I had a toddler with a soaked diaper, and no diaper bag. Luckily, the car had some extra diapers ready to go.
I would have gotten more points had I taken my own advice and had a fully clone of the diaper bag (something I keep meaning to blog about), which would have included an extra change of clothes.
Plan B was to hit Rite-Aide and buy some diapers and other supplies. But in this case, the trunk stash was faster and more convenient.