Sanding body surfaces (or any surface, for that matter) can quickly take its toll in sandpaper. "Veteran" sanders know that wet-sanding keeps the paper's surface clean for long periods, of course, but there are many situations where wet-sanding isn't appropriate.
When sanding body filler or fiberglass resin the paper "gums up" in no time, necessitating replacement. Whether you're using a pad, orbital or belt sander the problem is the same: the paper isn't worn out, just clogged.
To the rescue! Keep a wire brush handy when you are sanding. When the paper gums up on you, keep the sander running and press it down on the ends of the wire brush. The stiff bristles will dig out the filler residue and give the paper considerably more life before the grit wears away. Sometimes you can do this several times over.
When sanding fiberglass resin the paper gums up due to heat-melting of the plastic. You need to wait several minutes (15 or so) for the melted residue to solidify before cleaning it out with the wire brush.