Don´t be afraid to ask for payment up front. If you can get it, life will be a much simpler. Offer a small price discount for advance payment.
Payment through a confirmed, irrevocable letter of credit is another option. Before you accept a letter of credit, check out the creditworthiness
If you´re losing business because your terms are too strict, consider insuring foreign receivables. The premiums won´t cost as much as a deadbeat customer.