Maybe a little extra water to help the fruit size up? I know water's a precious commodity there but . . the big climatic hiccup has been the drought so . . if you can find a little extra water. I always water my fruit trees extra well if they have a big crop coming along. They're working hard to make all that fruit.
The other thing to be aware of is that most citrus have a limited life span, usually 20 to 30 years depending upon the variety. Sometimes a tree will put on an extra large harvest the year before it dies. Nature's way of assuring a new generation.
You said it was "old" but do you know how old the tree is?
