Here’s a 512px x 512px App Icon Template for general use. I built it on this template. I removed all the clutter surrounding it and kept it as a single file. Ideally you don’t want to have more than one image to edit.

Download Template PSD

To export for different devices, follow these specs outlined by Apple in their Human Interface Guidelines: Just go Save For Web & Devices in Photoshop and resize the image before exporting. Simple!
DeviceImage Height/Width
iPhone/iPod Touch57px
iPhone/iPod Touch Retina114px
iPad Retina?