One, Mississippi ... Two, Mississippi ... Three, Mississippi ...
But But but...
When you get to fifty, you've still only counted one, Mississippi!

For me, I try to count it as I had to sleep in the state for 2 nights, not necessarily back to back. The only state this has not worked out for me is Rhode Island. I guess I could do circles or take a rest day (cheating on the intent), but generally speaking, I have to be in the state for 2 nights. The reason is then it is a full day in riding in it.
For the NT Idaho panhandle crossing I mentioned, the "full day" riding would still apply (since we camped in Clark Fork the night before), but I'd still fail the two nights because we crossed the border into Washington that afternoon.
The 2 nights makes sense, and it's an easily applied, concise criterion. I guess my own is something like "substantial riding in a state" which I'll admit is a bit fuzzy. Most states are clearcut, a few have a mental asterisk by them, and then there's a couple that I'm not sure how they fall in the ridden - not ridden characterization.