1. Learn what your skin is like
First of all if you don't know what your skin type is, do some research or go to a cosmetologist for a consultation, they will be most helpful to you in identifying your skin type. Just a hint, there are 4 types to chose from: oily, dry, combination and normal skin. For example if your skin is very oily, choose for kaolin(clay) based masks as it will draw the excess sebum out of your pores.
2. Medical conditions, anyone?
Take into consideration the complexion of your skin as well as any existing medical conditions such as acne prone skin, sensitive skin, hyper-pigmentation, eczema, rosacea e.t.c. Any type of these are going to need a special care.
3. Buy things suitable for your skin type for the better effect
Look for products that meet the goals you want to achieve based on what your skin needs for the time being. If you want to reduce oiliness of the skin, look for masks that are made specifically for this situation. For example if your skin is oily and you want to reduce/control the oil, applying mask that claims that it will make your face shine like a disco-ball won't do you any good.
4. "This mask will transform you into a queen" ?!?!?!?
Read the claims on the package and don't expect miracles. If mask tells you it reduces the texture, you shouldn't be expecting the effects of a chemical peel. If it was possible, lots of people would not undergo the ridiculously painful procedures.
5. "Let's see what you're made of..."
Take a careful look at the ingredient list of the product you intend to buy. You don't have to be a chemist to understand what you see. Use existing websites that analyze cosmetics and provide you with detailed information on what certain ingredient is and for what skin type/condition it is meant to be in the product. My favorite two websites for the in depth analysis are:
www.cosdna.com/eng/ingredients.php
and
www.skincarisma.com