Reduce/Remove Sparks in AC and DC Contacts of Relays
There are a number of ways to do this, the most generic is an R-C quench circuit.  



You can even get them in one package.  People often use 0.1 uF and 100 ohms, but you may want to use trial and error to get the best results for your application.
If the load is at line voltage you might use an X2 for the capacitor for safety.  One capacitor company used to loan (sell?) R-C selection boxes to make this convenient.  There are also formulas published.