I bought the below case and NOW I can use it with one hand with out worrying!
Amazon.com: Shell Holster Combo Case for Samsung Galaxy S3 with Kick-stand -At&t, Verizon, T-mobile, Us Cellular and Sprint: Cell Phones & Accessories
MY REVIEW:
The Short: Got the case, and I'm surprised that I like it so much. I didn't expect a whole lot since it isn't some $50 case...but it's pretty slim and very handy.
The stand on the back can be used to stand it up both horizontally and vertically but I found a 3rd extremely practical use for it....you can use it to hook your middle finger in it while holding it with one hand. Using it in this manner allows a much more secure hold while using your thumb to navigate...this is particularly handy given that this is a big phone with a 4.8" screen!
I'll admit that it's slightly harder to get out of the holster with one hand than what I'm used to but I'm sure I'll adapt. The added protection and convenience of the multiple stand positions including the finger hook make this case worth the money.