Discover Copy Merged in Photoshop.
There is something immensely satisfying about discovering a Photoshop feature that's been sitting in front of you for years. The Copy Merged command has been waiting patiently in Photoshop's edit menu for well over a decade. Yet I've missed it all that time. A quick poll of savvy Seattle Photoshop production artists tells me I'm not the only one who missed this gem.
If you're working on a multi-layer Photoshop document and want to copy a portion of the combined image into another document, simply choose Edit > Copy Merged (command+shift+C) to copy a flattened version of your selection. You can paste anywhere you like — including a newly created file if you wish.
Source: This tip was featured on James Dempsey's terrific The Graphic Mac blog. To illustrate this technique, we used an image from last week: Prototype Packaging using Photoshop Smart Objects.