There is nothing you can do. There are various reasons for drop for example lack of proper pollination. Santa Rosa loves to drop in my climate, for five years it dropped almost everything on me. My tree was not heavily fruiting until it was 8 years old. I had no shortage of pollinating trees or bees. Do make sure there is a good pollinator nearby.

Scott

Thanks Scott! I have a small yard an the tree is next to peach, citrus, avocado, and apricot trees, all of which are blooming. I don't notice as many bees this year however- there were so many last year. Guess Ill have to be patient, it's it been in the ground 3 years :-). Looks like we might get some peaches finally, if the squirrels don't...

Do you have another plum tree? Santa Rosa will set some fruit without a pollinator but it will do better with another Japanese plum variety nearby.

Scott