Certain hostels give discount if you are an YHA member but they are generally the ones that have stupid things like curfews at 10.
things to buy:
Lonely Planet guide to Europe, or similar kind of book - gives you places to stay, places to go and see and lots of useful info. I'd say a guide book like this is essential in your group.
If you are going to get a lot of trains (they are quite shit in greece btw) then get the Thompson European summer train times book. (bout 13 pounds)
Websites to veiw:
www.hostelworld.com
www.bootsnall.com
Find a hostels own website off one of them and email them to book ahead - this means you don't have to enter credit card details into dodgy computers along the way.