Increasing pressure on the suction side is a good option. Positive Suction head by tank at height or pressurized tank are good options for that. Depending on the system it may be beneficial to reduce the temperature of the medium. Both work by increasing suction head, but neither may not be practical.
Temperature will be a nice additional parameter to consider. Indeed if the fluid is reaching its vapour temp, NPSH will be affected.
Inspection of suction line valves and strainers is a common practice that the majority is 1st thing to think of.
Reducing impeller speed will also reduced cavitation in pumps. Just be careful to make sure you still have the capacity for both pumping down the upstream side and supplying the downstream side.
On that note, how long has the system worked prior to these problems. It could be as simple as a clogged strainer or partially fouled valve on the suction side. If the system previously worked, I would suspect some sort of fouling.
Last thing, since this cavitation has occurred, has the impeller been inspected? It would be no surprise if it were pretty beaten if it went on long enough to fail seals. I might recommend completely overhauling the pump if it has been done already.
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