Rule number 1:
Remember the baby is not carrying the diaper bag. This means, that for the most part, save cutesy motifs, Winnie-the-pooh and other childish icons for the children’s clothing or kids luggage not the bag. you will be carrying.
Rule number 2
Parenting is messy. Very very messy. Hence, both the interior AND the exterior should be liquid proof, goo proof, sticky proof (your getting the picture). I know there are some nice looking bags out there with cloth or silk exteriors but I personally think that's asking for a big 'ol stain to happen. The Kate Spade bag on the right is one of my favorites for a blend of practicality and fashion savvy.
Rule number 3
Never trust the change pad the bags come with (unless they are very clear about the dimensions). They are always small or worse still attached to the bag! Is it just me or do most babies grow much longer than the length that the attached change mat allows. And, when is the last time any of the folks have been a bathroom in a restaurant?? The change table typically flips out from the wall and barely 
allows for the length of a squiggly infant never mind the bag! Ladies and gentlemen, cough up the $7-15 for a good, sturdy change mat, roll it tight and I promise you can fit it in any bag. In addition to the change pad stock up on a couple of key accessories - one of my favorite accessories is produced by Diapees and Wipees and holds a travel size pack of wipes and 2-4 diapers perfect for quick trips.
Rule number 4
Remember, once the baby is born, you will likely vanquish “the purse” unless you are on a girls’ nights out, or when the Nanny or grandparents can help. If you have baby in tow, its way easier to toss your wallet inside the diaper bag. This in my mind means that all diaper bags should easily double as hand bags hence – a compartment for cell phone, mascara and lipstick is essential. 
I personally love a small bag for makeup and an exterior pouch for car keys and mobile device. 
Rule number 5
Purchase more than one diaper bag. Four bags are not unusual. Here are my must have diaper bags: The every day bag( I lean toward designer diaper bags here); the daddy diaper bag (for when Dad's on duty); the day out bag; the overnighter or long haul bag. And, yes, in case you are wondering I purchase multiple tubes of my favorite lipstick, lip-gloss, mascara etcetera and keep mini-makeup necessities in each bag for mommy as well as baby! Trust me when I say the diaper bag is a worth while investment. Most kids are in diapers until they are 3, you might have more than one child so at minimum you are looking at 6 years of usage.
The Essential Diaper Bag checklist
For Her:
Extra pockets for cell phone or PDA pocket
Bags for her must have Matching makeup bag
Extra zip pocket for wallet
Solid slip free strap that will hang from the shoulder
Side pocket for keys and quick access to keep hands free
Bottle carrier optional (depending on the choice to breast feed I have found the bottle holder option unnecessary)
For Him
- Messenger bag or knapsack options (Eddie Bauer and Jeep make great selections)
durable fabric
- cell phone or PDA pocket
- Key ring for easy access
- Exterior pockets for quick access to wipes
- Bottle carrier
- Velcro openings
For Traveler/Road Trips
Travelling with baby requires an extra roomy size bag
Extra compartments for spare clothing
Room enough for a small blanket
Miniature bag or sac to carry the essentials quick changes in airports
Thermal compartment for milk, medicines or formulas
Multiple external pockets for quick access to soothers, blankets etcetera
Solid bottom (the bottom of the bag can take a beating when traveling with baby)
