This free tutorial uses Photoshop Elements, but can be done in any version, and creates a realistic silhouette effect against a sunset image
Start photo from Sxc.hu
We can afford to be rough with the initial selection. Use the Quick Selection tool to draw over each subject one by one to select them. If any areas need removing, just hold Opt/Alt and click over those parts.
Inside the Levels adjustment head to the Output Levels bar. Drag the white arrow inwards until it reads around 90. This will dim the subjects, making it seem as though they are silhouettes in your image.