+1 to a rack and bag that can clip on and off easily. I rode with a backpack for a while and am very glad I made the change. You can buy a decent rack for twenty dollars or less normally, totally worth it.
__________________
|^^^^^^^^^^^^^^| ||
|......GO.BROWNS........| ||'|";, ___.
|_..._..._______===|=||_|__|..., ] -
"(@)'(@)"""''"**|(@)(@)*****''(@)