I have several bags for my DSLR... a cheap one that came with it... that size wise was great, but all the plastic clips/latches broke within the first year...
the smallest I have is the older version of this where mine is all black but essentially the same...:
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http://www.dslrlab.com/review/lowepro-tlz-1/index.htm )
this is exactly the same as what my brother-in-law has for his DSLR, it's big enough to carry your DSLR and an extra lens or external flash in the outer pouch... I use one of the hidden film cannister elastics on the inside to hold my extra battery. also a zipped pocket inside good for lens cloth and SD cards.
I have a samsonite bag as well that has space for a lot more gear that I also have clip on backpack straps for... it's ok, but I found it uncomfortable to carry around at a festival for 2 days... however it did offer great protection for my gear from the rain as it has a rain hood hidden away in one of the compartments...
I typically don't bother carrying my external flash, but usually have my 18-55mm and 55-200mm VR lenses with the camera... depends on what I'm planning to do... sometimes I swap the VR for an old SLR lense I have for a more in between range... (yes I know that it's 1.5x on a DSLR...)
I think my next bag will be a Lowepro slingshot (single strap backpack) like this... I was looking at it before I got the samsonite, and changed my mind at the last minute... wish I hadnt...

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http://www.amazon.com/Lowepro-SlingShot-All-Weather-Digital-Backpack/dp/B000BAX50G/ref=sr_1_2?ie=UTF8&s=photo&qid=1251188931&sr=1-2 )