Hi Damo.
Same as Bill, but you can sometimes start with a sharp curved blade when the decal is large enough. Then sandpaper. To polish I use a kitchen/bathroom cleaning cream with an old toothbrush. Works perfectly on torsos and horsecoats. They may not be as shiny as before and the color may fade away a bit, but I usually customise those parts with my own decals and spread two coats of acrylic varnish (PEBEO) on top afterwards. The varnish actually makes the colours brighter, don't know why.
Cheers,
Arnaud.