Ok finding a hostel in Amsterdam during the summer can be very very difficult, especially when you are negligent in planning. Fortunately, people have found this hostel buisness to be very profitable in Amsterdam, as a result, many new hostels are opening there.
I was negligent and ended up staying at the Amsterdam-Hospitality. I LOVED IT! Great great administration. The owner, a young man with your interest in mind will make sure that you know his house is your house. He sleeps in the hostel with you. Pepe, a 24 year old Spainard assists and just chills with you. Ryan, the American perpetual stoned philosopher, makes you free breakfast and does your laundry for 5 euros. He is the most intriguing person you will meet in Amsterdam, I spent 8 hours at their breakfast table just talking to him: he is 25. The sheets here are clean, the place is clean, the shower is clean, it's very clean.
No curfew, laundry, breakfast, chill area, 12 beds only so nice group of people. If your idea of Amsterdam is just to relax and enjoy yourself in good company for 21 euros a nite this is the place.