About a year ago I purchased a great bag from
Lands end. It holds all I need for my purse and diaper needs plus it is water proof! I keep an emergency diaper bag in the back of our van with a change of clothes for the three youngest. I am trusting that Katy doesn't need them anymore. I also keep extra diaper and wipes, a first aid kit, spoons, napkins, disinfectant, an extra towel and washcloth, and a few snacks (which are probably stale by now). Now Kris will tell you that it isn't always stocked appropriately, but I do make diligent attempts at keeping it stocked when something gets used.
So I keep as little as possible in my diaper purse. In a little removable zipper bag I keep 2 diapers for Jay and Klaire, wipes, and diaper cream. Klaire and Karolines sippy cups usually make in the bag too. Now on Sundays, I do bring small take along bags for Jay and Klaire with the essentials. Getting rid of the all the extra that never seems to get used has certainly lightened my load.
Update: 1/2012
I now only carry diaper items for Jay. I do have a small backpack for Jay when we are at church and I have to leave him, but that is all I carry. My purse works to house everything else I need when we are out and about. Having the arsenal of extra's in the car works best for me. The key it keep it stocked.
I mean do you really need 10 diapers, 3 changes of clothes, powder, bottom cream, baby wash, hairbrush, sunscreen, nail clippers, 4 paccies, and wipes to just go to the grocery store or church! It took me awhile, but I learned.