I use my cell phone to take a picture of my plate, especially if I am eating alone. At times I am truly amazed at the amount of food I have on my plate & immediately divide it in half! At other times I am proud of the proportions and color range of food I have selected.
If I find I am consistently over estimating the proper portions I go back to serving myself in recycled frozen dinner trays that are the three divider kind. A piece of fruit & a glass of water & I am back on track in no time!