This is an awesome tutorial in adding realistic light and shadow effects on images using brightness, contrast and gradient. A piece of advice for those who just want the actual tutorial, skip the first three minutes of the video.
Update: I just want to include the final episode of YouSuckAtPhotoshop. The tip here is similar to the first episode and I included it here just to complete the entire season. Dane Cook was behind You Suck at Photoshop all along. He’s in this episode. Thanks to Oliver for the info and thanks to MyDamnChannel for the great tips.