I tend to go seasonal with bags, but I usually change them out more often. I have ... well, a lot. Bags are more of a weakness than shoes with me.
I carry wallet, keys, a book (always), a pen, usually my datebook, my MP3, a little cloth zip bag with mints and cough drops, a small makeup bag with tampons, safety pin, tissues, lipsticks, nail file, and mirror, and then cigarettes, lighter, and cell phone.
If I have a bigger bag, more stuff goes in it.