[Peter Zack]

As JC Sullivan suggested focus is critical and a good Macro lens might be just what you need. The Sigma can do a very good job most of the time. Are you using a tripod?
Here's what I'd suggest.
Tripod
Cable release (or 2 second MLU) to eliminate shutter vibration
change backgrounds with different colours or reflective surfaces. You can take the back side of gift wrap paper which has a slightly reflective surface and get and nice refection to highlight the product. Also use a single colour for the back and bottom and put a piece of glass under the item. Make it POP with reflections. Darker items will look good with lighter colours but try something other than white. Light items look great with black/blue/burgundy etc.
Shutter speeds are not as important as aperture when a tripod/cable release is used. Learn to control th DOF to blur the background. Open the lens up enough to get the item sharp but the rest blurred.

[Peter Zack]

This might work. I used blue gift wrap paper and some of my old socket heads (didn't bother to clean them for this test). You might prefer some different fabric colours that are cheap at fabric stores. Using a 3 light set up and light box. This was shot with the very same lens you have Sigma 18-50mm f2.8 (non macro). I find it to be a very good and sharp lens. Gets close enough to get a sharp shot. Metal can be an issue because the surface seems to have a softness when you get up close.

Cropped 50% from original.

[Peter Zack]

And this was done with the FA50mm f2.8 Macro. Again cropped about 25%

[dhulsmeyer]

ha. That's the same lightbox I use. I got the idea off strobist.

[Peter Zack]

Finally using the Sigma and glass under the items.