You don't even want to know how long I've been trolling the web trying to find a easy way to do App Screenshots in Device Mockups.
It's not that it's particularly difficult to do in Photoshop, it's just tedious, very annoying and certainly not a perfect science. This is where a service like, MockUPhone comes in very handy.
All you have to do is have your app screenshots with their correct dimensions ready, choose your device, upload your screenshot wait about 3 second and your done.
You get a instant preview of your screenshots, plus a zip file to download that contains a folder with all perspectives.
I did some screenshots from this is for both the iPhone 5 and iPad Mini. Check them out below.
This is usually a 10 minute task in Photoshop, just for one perspective.
Check out MockUPhone.