Pack your diaper bag every single night. This was a hard won tip. Took me forever to figure out that you can't just throw baby in the car seat and hightail it out the door.
You need diapers and wipes and a bottle (if that's your thing)/sippy cup, and a paci (if that's your thing)/lovey/best friend that they won't leave the house without. And you need your wallet and your keys and their health card (which you may or may not have lost twice...ahem), and a few toys to keep them occupied, and a couple of spit-up cloths, bibs, blankets. You also need your cell phone. And a snack. And extra clothes incase there's an explosion.
And you can get none of this ready while baby is crying, pooping, whining, spitting-up, etc... And babies will always do those things when you are desperate to get ready.
So, I pack the diaper bag the night before- every night- whether I have plans or not.
I make sure there are a minimum of three diapers (per kid), a good amount of wipes, and an extra outfit. I also make sure there is an empty plastic bag in case I need to throw out/contain anything that is pooped on, peed on, leaking, half eaten, etc... and an empty ziplock which is awesome if someone suddenly needs to throw up (those 9 months of morning sickness taught me something).
I have standard diaper bag toys that stay in my diaper bag. I keep a paci in a ziplock bag inside my diaper bag. I have 2 clean spit-up cloths, one bib, and one blanket.
I keep my wallet (with both boy's healthcards...lesson learned) in the front of my diaper bag. I always have extra cash stashed in one of the pockets...just in case. I also fill up a ziplock with snacks for Z (usually goldfish crackers or raisins or an apple) and a granola bar (in case I'm starving). Lastly, I keep a ponytail holder jammed in a pocket because I always need one and a tube of concealer....for obvious (sleepless) reasons.
In the morning, if we're going out I throw in a bottle of formula for W, grab my keys & blackberry, and we're out the door. Literally take 5 minutes.
Yes. It's annoying to double check the diaper bag every night but the time it saves me in the morning (or afternoon) when we're trying to get out the door and both boys need their diaper changed and Z is running around refusing to put on shoes and W is screammmmmming at the top of his lungs because he HATES being buckled in his car seat. Well, those are the moments I am happy I took that 2 minutes to double check that my diaper bag is stocked and ready to go.