Ok first what are you trying to show with your picture. I mean I see a slightly wrinkled pink shirt on a stand with a geometric design. Are you looking for more pop or crispness to the picture? Sorry Im not sure what your trying to achieve. Maybe if you explained it a bit better.
I think you should make the wall pop more, everything is kind of dull but I feel if you just increase the brightness, the light pink shirt will be blown out. I don't see any blur. I'd say instead of trying to fix this one, re-shoot and maybe you could straighten the shirt on the mannequin/pole. Try positioning your light [flash?] so that there is not shadow, kind of takes away from the photo for me.
And yes, ironing the shirt would have made an impact.
If there's low light, use a tripod.
Was this taken with a P&S or DSLR? I would try manually focusing if you think it's still blurry.
*Look at some clothes websites and see how they have their light and backgrounds.
*What's your focal length?
Moving the mannequin away from the wall will be a start at helping to get rid of the ugly flash shadow. Aside from that, the shirt doesn't fot the mannequin very well which doesn't make it look very flattering.
To improve the look of the photo you need better lighting mainly. I think you mentioned in another thread you have SB-600s (plural?). Two of these is pretty close to ideal really, but you'll need some cheap wireless triggers and two stands with shoot-through umbrellas, set one up either side for even lighting (to start with anyway) and you should have 10x better shots.
Also, a tighter crop is needed on the above shot. But I wouldn't bother about actually editing it, get the right gear and then re-shoot.
Second the black background and pull your object out from your background to give it some depth. Try the reshoot and dont forget to iron your shirt. Lets see what you come up with.
I will be shooting a bunch of different kinds of materials and colors. Do you think a black background is better in general or just for this particular shirt.
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Your subject should be at least 4 feet from the background, 8 feet would be better.
You need at least 2 lights.
The bigger the apparent size of your light source, the softer the light (and the shadows) will be. Softer light makes your product look better.
If the overall length of the t-shirt is say 36 inches you need an apparent light source bigger than that. An inexpensicve way to make a SB-600 or two seem large is to shoot it/them into a photographic umbrella. 45" umbrellas will work ok for t-shirts.
Get the speedlight off the camera (OCF, off camera flash).