By far the best and cheapest way of spending money abroad is by the "no load/commission" credit card, but only if you pay the balance off in full every month. With this type of card it's far cheaper to spend on them (tavernas and supermarkets) than withdraw cash and spend it.
It's likely that you will only get one of these cards if you have a good credit rating and are the type who does not go into debt, otherwise it's the pre-paid card to go for.
The current best at present is the Halifax Clarity Card, you don't have to be a Halifax customer to apply for one.
There is no foreign exchange fee worldwide and there is no cash withdrawal fee, unlike most other cards, however you will be charged interest from the date you withdraw the cash (the same as all other cards) but fairly low interest at 12.9%, so set up a direct debit to pay it off in full every month.
There are extra benefits if you have a Halifax Reward current account
There are a couple of others which are nearly as good if you have them, Post Office (£3 cash withdrawal fee), Nationwide (£2 cash withdrawal fee) and Santander Zero (no cash withdrawal fee) but no fees on any of them if using in tavernas or shops.
A lot of pre-paid cards have start up fees, ATM withdrawal fees (FairFX is 1.5 € for each withdrawal), inactivity fees (both My Travel Cash and Travelex charge a £2 monthly inactivity fee once a card has not been used for 12 months. FairFX and Caxton FX do not)
Another thing to remember is if the retailer gives you the choice of paying in £'s or euros, always choose euros or you will lose out.
Last year only one of the several tavernas and supermarkets would not accept any type of card, The Beachcomber.
In 2012 I was getting about 0.05 to 0.06 euros above the San Stefanos exchange rate, food for thought.