Handbag And Purse

Use the inside of a banana skin then buff it with a soft cloth.
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you could try a small amount of baby oil. you might want to contact someone who knows about cleaning leather. like a dry cleaners.
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Armour All for leather, found in the automotive departments (U.S.) It works great on leather seats, I am sure it would work for leather bags as well. And it is cheaper than the leather cleaner they try to sell you in the department stores.
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There are lots of different sprays availiable for cleaning leather bags etc, don't know what part of the world you are from but the product i use is called "son of a gun "you can buy it in car shops & some super markets.
It wont leave any greasy residue like most products & it will look like new in no time.
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It really depends on the type of leather. You should check out this guide:

http://community.livejournal.com/coach_lovers/548095.html
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Either a baby shampoo or saddle soap work really well on leather bags.
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