Clean the clippers with soap and water
Dry them completely
Remove rust with fine sandpaper or a metal file
Sharpen the cutting edges with a nail file or sharpening stone
Tighten the pivot screw if it is loose
Apply a small drop of oil to the pivot
Realign the blades if they are misaligned
Replace a bent or broken spring
Replace the clippers if the blades are cracked or too worn
Test the clippers on a fingernail after repair