Target Australia has a tutorial on neatly folding elasticated fitted sheets, something that’s always stumped me:
Step 1 Hold the sheet inside out, by its two adjacent corners on one of the shorter ends. Position your hands inside each of these two corners.
Step 2 Fold the corner in your right hand over to the corner in your left, enveloping it. With your right hand, pick up the corner that is hanging down in front and fold it over the two corners in your left hand.
Step 3 Pick up the last corner and fold it over the other three corners. The sheet should now be right side out.
Step 4 Place your folded sheet on a table and straighten it, tucking in the elastic edges as you go.
(Thanks, Feren!)