The first step I would advise would be deciding upon a specific background size to be used for each image (perhaps 600x400; whatever). Then use that same size for each of your product images.
Next, I would consider using "Guides". By setting up a vertical and horizontal guide where you want your items to be located, you can ensure consistent placement.
For instance, if I wanted to follow the rule of thirds and place all of my items on the right side of the background, I would setup guides in the following manner:
Under the "View" menu, select "New Guide..."
Set a Vertical guide to 66.67%
Repeat the step above to create another "New Guide..." and set a Horizontal guide to 50%
You should now see two guides forming a cross-hair which will be used to ensure all images are aligned to the same point.
Go back to the "View" menu and verify that "Snap" and "Snap To > Guides" are both activated. This will ensure that when repositioning images they will automatically snap to the cross-hair.
The final step is to actually reposition your image by activating the appropriate layer then use the move tool to drag the image over to the cross-hair. The mid point of your image should "snap" to the cross-section of the vertical and horizontal guides.
This is just one idea. No doubt more experienced users will be able to provide others.
