The first thing to look for are no foreign transaction fees, these can be between 3-5%. Second, you need to look for a debit card that does not charge for overseas ATM withdrawals, as these can be $5 per transaction plus 3%. Therefore a cash withdrawal of $300 will withdraw $314 from your account.
Third, you need to look for a debit card with no monthly fees or account keeping fees. Finally you need to choose a debit card with good security and is easy to transfer money into while you are travelling overseas.