What works for most of my sanding including sharp corners is to make sanding "blocks" from thin styrene stock, .010" - 020" depending on how flexible you like. Most of the time I'll use a 1" x 2" size, which fits my hand, sometimes bigger or smaller for specific tasks.
I like to make them in bulk, cutting 1" x 3" strips of 400, 600, 1200 wet & dry sandpaper, then glueing them to the styrene with thin rubber cement. Any craft cement that bonds parper to plastic works as well, like Ailene's Tacky Glue for example.
These little sanding pads can be used wet or dry, and conform to curved surfaces. The folded over ends are really handy for tight spots.