Urinary blockages

Has anyone any idea how to prevent these problems in castrated cats? Spirit
will be the first castrated male I've had since the 1960s. I had a male with
endless urethral blockage problems back then and I vowed I'd never adopt
another male cat or have one "fixed." Now I find myself probably keeping
Spirit who will be fixed at the vet clinic tomorrow. What is the recommended
diet these days to help prevent his painful condition? I know the vet I
worked for in the 70s believed it was due to poor diet and secondarily by
the castration itself causing the urethra to shrink and narrow. Any ideas?
Thoughts on this? Advice? I don't want Spirit suffering like that cat did
for years.