Almost a year and a half into my career as a bus parent, I'm still struggling to identify the best, most-efficient, most bus- (and Bus Chick-) friendly way to transport our stuff. I need a setup that does not have to be altered significantly from day to day or trip to trip, an easy way to access important items (like my umbrella, bus pass, and phone), and the ability to easily separate my stuff from Chicklet's -- for those times when I'm traveling without her or she's traveling with her dad. I need a bag that looks reasonably professional but that is big enough to fit all of our stuff -- and still reasonably comfortable to carry with a baby (er, toddler) in a carrier. I think I've settled on getting some kind of insert (which I have not yet identified but believe exists) for Chicklet's stuff. It would make separating (and co-parenting) easy, and would finally return my bus chick bag to my complete control, to organize as I see fit.
Which reminds me: If the last six years have taught me nothing, it's that every transit rider's bag should contain at least one empty plastic bag. Almost any medium-sized plastic bag will do, but the sturdier it is, the more versatile it is.
This may seem like an unnecessary or frivolous addition, but its uses are almost limitless. A plastic bag can be used to cover a wet or dirty bench (or other potential seat), or to protect a not-so-bus-chick-friendly 'do from bad weather. In the event of sudden illness or motion sickness -- or when busing while pregnant -- it can be used as a public transit version of an airplane barf bag. If you find yourself near a store during your travels, it can be used as a shopping bag. It can carry a wet hat or mittens or umbrella. Bus Nerd carries a reusable coffee cup with him in his bus bag. (He uses bus waits over 10 minutes as an excuse to buy a vanilla latte from the closest coffee shop.) After he's used the cup, he wraps it in one of his plastic bags to prevent any remaining liquid or odor from leaking onto the other contents -- namely, gadgets and wires -- of his bag. And since we're talking kids, a plastic bag can also be used to transport soiled cloth diapers in pinch -- though I don't recommend it as a permanent solution. I have a special, zippable lined bag for this purpose. (I'm also fortunate that Chicklet is mostly potty trained.)
Do note: A single plastic bag cannot be used in all of the previously described scenarios. Several are single-use cases and require the bag to be thrown away and replaced. If you find yourself replacing your bag often, you should consider carrying more than one. Given that they're virtually free, easy to replace, and take up very little space, you might as well.