Clean the tweezers with soap and water or rubbing alcohol
Dry them completely
Inspect the tips for damage or misalignment
Use a fine nail file, emery board, or very fine metal file
Gently file the inner gripping edges of the tips
Keep the original tip shape while filing
File only a little at a time
Test the grip on a thin hair or small piece of paper
Repeat filing and testing until the grip improves
Use fine-grit sandpaper for a smoother finish if needed
Wipe away all metal dust
Realign the tips if they do not meet evenly
Replace the tweezers if the tips are bent, chipped, or too worn