The American Dietetic Association recommends the following visual cues to help keep track of portion sizes.
1 teaspoon butter: tip of thumb to the first joint
3 ounces meat: deck of cards
1 cup pasta: tennis ball
1/2 medium bagel: hockey puck
1 1/2 ounces cheese: three dominoes
2 tablespoons nut butter: ping-pong ball
1/2 cup vegetables: light bulb