Many of us own a five-in-one reflector of some sort, as they're cheap, versatile, and effective instruments. However, they're commonly misused, and this great video will help you learn to use them effectively to get the best results.
Coming to you from David Bergman of Adorama TV, this helpful video talks about the proper way to use reflectors. Both studio and natural light photographers frequently use reflectors to either kick back a bit of light to bring up shadows or even as a key light if they're working outside. The mistake I very commonly see is a photographer using the silver or gold side of the reflector and leaning it against their knees or having the subject hold it. This often ends up kicking too much light back onto the subject and lighting them from underneath, creating a bit of a horror movie effect. Rather, as Bergman details, one should think about the reflector as its own light source and position it the way you would a normal light. If you want to fill in shadows a bit, consider using the white diffuser panel for a subtler and more pleasing effect. It can change from situation to situation, so watch the light on your subject's face carefully. Give the video above a watch for the full rundown.