Try changing your sanding motion from back-and-forth to small circles. It spreads the pressure around and can really save your joints. Your hands will still get tired, but they won't hurt as much.
Seriously? @thomascooper I've sanded plenty by hand and never had joint pain bad enough to change my whole technique. Maybe if you're doing it for hours every day, but for a weekend project my wrists and elbows are fine.