Two ideas I have had:

Make spaying and neutering a tax-deductible expense.

Also - I tried this in Ontario once - how about the vets having gift certificates for spay/neuter only? (as in, it's not money you can use for cat food or etc).
This would make great gift ideas!

In ON, I was told that the head Vet organization - whatever it was - didn't approve of such things, because it would create competition.

This was 20 years ago...perhaps times have changed.

I know we have a couple of lawyers on Pet Talk, and legal assistants who make a good number of legal eagles all together, and the more practical suggestions we can make, the better.