I know it is a pain to do but measuring everything helps me instead of eyeballing portions. I was putting too much cream in my coffee on the one hand and then ripping myself of in my cereal portions on the other
Here are some tips for portions:
3 oz steak = a deck of cards
4 oz poultry/fish = the size of a large bar of soap
1 cup cooked pasta = the size of a tennis ball
2 oz bagel = the width of a credit card
1 oz cheese = the size of a domino
2 tbsp peanut butter = one ping pong ball
3" potato = computer mouse
As kellys pointed out, always read your labels! The package might say 30 calories per serving but never assume that the serving is the package. If you read the nutritional information you often find that the serving size is less than the package contents.