Use APIs in ways that add value to the provider (i.e. you're paying them or adding value for their users). If it's in their interest to keep you alive then then they're most likely to keep the API going.
Sometimes the terms might be more restrictive than is actually enforced. Doesn't really help with your fear, but if you avoid undermining them in every way you can, it's probably less likely that they'll try to cut you off. Having a backup plan (Google Local API?) probably isn't a bad idea, though.
Contacting them is your best option. Find some one to talk to, explain what you'd like to do, and how the terms restrict you, and see if you can't find a way to have an exception, clarification, or some other way around it.
I "love" clause 6D:
"You agree that you will not, and will not assist or enable others to: use the API on behalf of any third party;"
So in other words, no one can use the API except to get Yelp information about themselves.
What a fucking joke.