To begin with, the P5+1 could adopt the very successful style of negotiation practiced by Palestinians as well as Iranians. This is to whittle away at the position of the other side by extracting one little concession after another, but then to delay the negotiations indefinitely when the deal seems to be imminent. The result is that when negotiations do resume, it is not from zero, but from an inferior initial position of the other side.
Whenever a deal seems near, one of the P5+1 should come up with a further demand or demands. What they could do is adopt that role in succession, so that Iran is the party that needs to keep starting afresh from a worse position.