Choosing the right bag: The bag I like most has 5 pockets, one in front with smaller pockets inside (for small items like vitamin d and bibs) A large inside pocket for all your linens and bigger items and another outside pocket for a changing pad and two on either end that I keep a room temp bottle and the other is for my cell phone or wallet.
I put a check-list in the side pocket with things I need to bring for an 8 hour day trip.
2 4oz bottles cold or partially frozen
1 4oz bottle room temp
2 burp cloths (I use gerber cloth diapers)
10 disposable diapers (if you use cloth bring 10 inserts and 2 covers)
whole pack of wipes (I put them in a zip-lock so I don't have a giant box taking up space)
Waterproof changing pad
Small tube of rash cream
Rattle or noisy toy (he likes the telephone shaped rattle)
Small quilt or blanket to use as a play mat
2 short sleeve onesies if summer (longsleeve if winter)
2 sleepers with feet
2 swaddle blankets
I am probably forgetting something now that I'm writing this out (and watching 'I didn't know I was pregnant') but the point is OVER PACK! The unexpected always happens!