What's a Serving?

I've been trying to cut back on how much I eat. But in a day and age where it is more common to eat out, I'm not very good at judging a serving. I'm finding it handy to compare food to other objects to judge the serving size. For example, the recommended amount of meat for a healthy meal is 3 o 4 ounces - about the size of a deck of playing cards. Some more examples:

1 oz. meat: size of a matchbox
1 oz. cheese: size of four die
2 tablespoons of peanut butter: size of a ping-pong ball
1/2 cup of pasta: size of a tennis ball
1 medium apple/orange: size of a tennis ball
1 cup of vegetables/fruit: size of a baseball
1 medium potato: size of a computer mouse
1 cup of lettuce: four leaves

You can get a wallet-sized portion guide from the American Cancer Society at:
http://www.cancer.org/docroot/subsite/greatamericans/content/Portion_Control_Guide.asp