There are four factors that you should not neglect when choosing plastic frames:style, color,craft and label.Choose one that is stylish currently.Keep the frame flat before your eyes to check if the color of arm are different.Open and close the arms to feel its flexibility.
In fact,I think there is not much difference in choose plastic frame or mental one.When you are choosing a plastic,trying it on is much necessary to see if it matches your face and can be placed correctly on your face,whether the point of support is stable and if it is comfortable to wear etc.Also you should check it's workmanship of welding spot,symmetry ,gloss,flexibility of arms and so on to assess its quality.
There are many types of plastic frames for you to choose. Before you select one that suit you fine, you should know your face shape,skin color and your own preference. Different personal style needs different kinds of frames.You'd better try them on when you begin to select your plastic frames.