what has to be true for ASIC SHA256 mining to work on a coin?

[Cryddit]

ASIC miners work on alts whose blocks conform to some set of rules.

What -- exactly -- are the rules?  

I want to add new fields to the block, new transaction types, and a couple of other things - but when I'm done I want the result to be able to take advantage of all that ASIC infrastructure out there.   So what kind of binary changes can I make and what kind of changes will result in a block that can't be mined with existing ASICs?