I do have one "real" diaper bag... a skip hop 25 dollar dealie from Target. But over the past 4 years I've used all sorts of bags! I carry my bag everywhere and it holds everything so it needs to be something that says "me"... not something that says "diapers", you know? My favorite was a canvas messanger bag with a huge skull/crossbones on it but I passed it along via Holiday Helpers a few years ago.
And since I like to change my look that plays into it too... usually I have the actual "diaper and diaper stuff" in a roll kit I made years ago (we sposie while out of the house so it's a bit less bulky than it could be). Basically a long rectangle of fabric folded over and sewn to create pockets big enough to hold diapers for each kiddo (always one more than you think you'll need on a trip), a thing of wipes, and a pocket for mama stuff like pads/nursing pads/chocolate bar. The whole thing folds up and ties with a ribbon and when unrolled all the way doubles as a changing pad. So that little diaper roll can be moved from bag to bag, turning whatever bag I choose into a "diaper bag".
From there, you just need a bag that will meet your own needs. Room for a change of clothing for the babe (and a top for you too), space for a bottle if you are bottle nursing or have a sippy/sigg for an older child, maybe space for a ring sling or a receiving blanket (those things can do all sorts of double duty), etc.
I've found that for the first few months a newborn on the road needs a surprisingly large amount of "stuff" because diapers leak, spit up happens, they poop every single time they nurse, and so on. Then when you add in the random stuff for the older kiddos (a book here, crayons and paper there, a sweater since dd1 refuses to put on on till she is purple, some allergy safe snacks, etc) and the stuff for me (book, snack, clean/dry nursing top in case the new babe urps all over the one I'm wearing or I leak, water bottle, cell phone, wallet, keys) it's a pretty big pile! So I use a bigger backpack that I can wear and access easily while slinging the babe and juggling the older kiddos. But then after things calm down a bit I can downsize the bag... fewer dipes, fewer clothes, less stuff in general.
Oh, and I know it's not really "green" of me, but I've found that a nearly limitless supply of ziplock bags can make life on the road easier during those really messy first few months.
