When designing an icon in Photoshop (or your illustration app of choice), a neat way to preview it downscaled to its intended size is to reveal a sliver of desktop behind the window and save your working copy there.
If you have icon previews enabled, you should see a nicely resampled image that updates every time you hit save! You can resize the preview by tweaking your desktop icon size (ctrl+click > Show View Options).1
I imagine this tip would work especially well if you’re lucky enough to have a HiDPI monitor. I still use Iconsider to preview icons on my iPhone’s retina display.
You can do something similar within Adobe applications via Window > Arrange > New Window for X but zooming right out results in poor resampling and loss of detail. ↩