For water soluble contaminants, rinse probe in deionized (DI) water.  If ineffective, soak probe in warm DI water with household detergent for 15 - 30 minutes.
For oil-based contaminants, rinse probe in ethanol or acetone for short (5-minute) periods.
After cleaning, rinse probe in DI water to remove residual cleaning reagents.  Perform a meter calibration before proceeding with sample analysis.