1) if you find you lose definition once an icon has been resized down to 100x100, don't be afraid to redraw bits of it at the smaller size. no matter what size I drew my art at, or how clear my lineart was, I always find SOME part of the icon will look blurry or non distinct once I size down. here's what I do: zoom in very, very close (normally to around 300%), grab the Pencil tool, and start recolouring individual pixels, or stroking over my lines in a low opacity (around 40%) depending on what the icon needs. it's surprising how a slight change to just a few pixels can sharpen your icon and make the facial expression more readable. it takes some practice, but you'll get good at it. constantly zoom out back to 100% to check how things are looking, and do all your adjustments on a new layer.
2) icons will often need sharpening, but have you noticed that when you Sharpen and icon, you'll get a light ghostly line around all the edges? you may want to use Sharpen to make your lines crisper, but you ALSO want to keep your colours lovely and flat. assuming you're working in flat color, make a new layer and set the opacity to "darker color", then colour over the pixels that got lightened. it will make your icons look a hell of a lot better, I promise. (the principle is still the same if you work in a style other than flat color, it's just more fiddly).
3) when using Transform to resize your art to fit within a 100x100 canvas, try and leave at least a few pixels "bleed" at the edges. what I mean is, don't let the outer edge of the art sit flush within the edges of the canvas. why? because photoshop does this annoying thing where if you resize an image sometimes it'll lighten the pixels at the very edge of the image. if you DO get this, you can easily color correct it using the same technique from point 2, but it's easier to just avoid it entirely.
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2) icons will often need sharpening, but have you noticed that when you Sharpen and icon, you'll get a light ghostly line around all the edges? you may want to use Sharpen to make your lines crisper, but you ALSO want to keep your colours lovely and flat. assuming you're working in flat color, make a new layer and set the opacity to "darker color", then colour over the pixels that got lightened. it will make your icons look a hell of a lot better, I promise. (the principle is still the same if you work in a style other than flat color, it's just more fiddly).
3) when using Transform to resize your art to fit within a 100x100 canvas, try and leave at least a few pixels "bleed" at the edges. what I mean is, don't let the outer edge of the art sit flush within the edges of the canvas. why? because photoshop does this annoying thing where if you resize an image sometimes it'll lighten the pixels at the very edge of the image. if you DO get this, you can easily color correct it using the same technique from point 2, but it's easier to just avoid it entirely.