i bank with barclays, and do not have an overdraft on my account, i checked my balance on a cash machine spent less than what it said i had available and somehow went overdrawn and got charged for going into an unauthorised overdraft, after speaking to them and questioning them as to how if i haven't got an overdraft they carried on letting me use my debit card when i had no money in my account, to which they replied that companies such as tesco have a guaranteed withdrawal scheme so you dont get turned down when you shop there and the bank cant stop it happening.
Barclays were brilliant with me and waived the charge because it had been the first time in 20 years it had happened, they offered me advice on managing my account and ways to avoid going overdrawn again which happened again the following month and i only got charged the standard barclays fee of �8 which is very low compared to the likes of halifax, abbey and lloyds who charge �30+
my advice would be work out how much you can afford to spend each week after direct debits have gone out and keep the amount in cash in your house and dont use your cards, once the cash is gone you know you cant afford to spend anymore