Hi Sue,
First I guess I would ask what item you want to center? For towels, fold in half length wise and mark with a pin, that would be your true center for the. Articles of clothing are bit more difficult. I have found that the embroiderers buddies work well for shirts, blouses at the chest area, or left or right of the chest. Again it just depends on what you want to center.For the back of a shirt, I simply fold in half and mark it with a pin, then I measure to the right and left to check that I have true center. Once you have done a few times it becomes easier.
Here are some guidelines for you to follow for putting designs to the right or left of a shirt or blousez;
for a man'sXL &XXL 9" down from the left shoulder and 4.5" right of center
Women's S<M<L 6.5" down from left shoulder and 4" right of center
XL&XXL 7" dow from left shoulder and 4" right of center
Childs 4"-6" down from left shoulder seam and 2.5 - 3" over from center
Hope this helps and I know many will have some wonderful and fabulous tips for you 
Judy 
Help me to start each day with a new attitude and plenty of gratitude!