Hand sand to 600 grit satin or higher, make sure you get all the scratches from the coarser grits out. Use a buffing wheel (6 inch or bigger usually works best) loaded with green buffing compound (chromium oxide, available at most hardware stores and online woodworking/knifemaking supply shops).
Edit: Aus-8 will take a mirror finish. You can actually get a mirror finish at as low as 220 grit but it will not be top class, in order to get the 220 scratches out with the buffing compound you'll likely have ripples and uneveness in the mirror finish. Definately make sure they're not swirls though, all the scratches need to be in one direction, this A: polishes easier and B: if you do decide to just stop at a lower grit before buffing it doesn't look as hideous.
Oh, do be careful if you use a buffer, they've been known to injure many a knifemaker who was distracted for a moment. They can grab the blade and send it flying if you aren't careful. If you don't want to use a buffer then I suggest you satin finish (the right way, no swirls!) to 1000 and then once it looks good use one more piece of paper till it's worn smooth and won't cut at all, this will leave the surface almost mirror polished and a bit of hand polishing with simichrome/flitz/metalglo will get you the rest of the way. Use a sanding block to make sure the surface is being sanded perfectly flat and if the knfie is hollow ground a large pink eraser is one of the easiest ways to keep the sandpaper backed on the hollow grind.
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