It must have a strong geopolitical drive and maintain political solidarity with some of the other members. It must be big, influential and its economy must be intermeshed with the economies of the others. It is better if a monetary union is preceded by a political one. Wage and price flexibility are sine qua non. They can mitigate and ameliorate - but not solve.
Transfers also call for a clear and consistent fiscal policy regarding taxation and expenditures.
Problems like unemployment will plague a rigid, sedimented union. The last prerequisite is clear convergence criteria and monetary convergence targets. Judging by these requirements, the latest monetary union did not learn sufficiently from the mistakes of its ill begotten predecessors. It is set in a Europe more rigid in its labour and pricing practices than 150 years ago, it was not preceded by any political amalgamation, it relies too heavily on transfers without having in place either a monetary or a fiscal policy to talk about. Without correcting its ways, this monetary union will join its forefathers and remain a footnote in the annals of economy.