I use a Bagman support:
http://peterwhitecycles.com/saddlebag_support.asp
Sorry- they are currently out of stock, but you get the idea.
When you have big saddlebags, they tend to sag after a while, especially if made of canvas or soft material. This is more of a problem with women's bikes because there is less space between the saddle and the back wheel- women's sized bikes are less tall in general than men's. A rear rack or support keeps the bag supported and well off the wheel.
I also have a small bag with just enough room for spare tube and tools, a granola bar and a cell phone, etc:
http://www.velo-orange.com/saddlebag.html
I like this one very much (but I usually haul too much stuff around for it to handle)....and the top outside strap can actually be used to secure a rolled up lightweight windbreaker or extra shirt. This bag does NOT need an additional support under it- it has a lower buckle that straps around your seatpost. I think it's rather handsome! Size wise- you could probably cram a grapefruit into it but nothing additional. My DH uses this one sometimes nowadays.




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