These are some tips that I got from a friend of mine who loves to switch purses.
In my purse I currently have:
-2 sunglass cases - One has my sunglasses and the other has my ipod and headphones
-In my front pocket, I have lotion, chapstick, cell phone
-In the inside pocket, I have face powder and medicine in a case
I like to keep it clutter free but also easy if I need to switch purses. I also like to buy purses with some organizing compartments. REI has great purses for this - they aren't fancy but they are practical. (http://www.rei.com/product/763408) Something like this.
My friend keeps her stuff in small make up bags so she can take them out and put them in another bag in seconds.
There are some cool purse organizers out there now but I thought the make bags was an inexpensive way to switch back and forth from any size purse.