The principal existing resource are the youth hostels, either belonging to the hostels of Hostelling International or local organisations. The hostels of Hostelling International offered more guarantees, but they are not always the cheapest. You should never pay before you see the facilities since after making the payment you might not be able to get a return.The Hostels have the advantage of having generally good conditions at a low price and usually exist in any city. The biggest drawback is that they are extremely crowded, especially in areas of greater influx of tourists. Sometimes it is almost mandatory to arrive early to get beds in most tourist places you need to be there before 9 am!Now is also common to see "salesmen" for guest accommodations in train stations. This usually happens on trains stations in the east . Usually these people exposes brochures of lodges for which they work, inform about the prices of each and sometimes offer free transportation. When properly identified and to "look" reliable, it is one option to consider, often becomes cheaper than many youth hostels.
Another possibility for accommodation is camping. They have the great advantage of being much cheaper, but of course there is the hassle of carrying the tent. Moreover, many of the camping sites are usually far from urban centres, so you have to add the price of the transportation in the accommodation (in Amsterdam is worth the difference!)
Besides food the biggest slice of the budget goes for housing. Hence one of the best way to lower costs is to travel by night on the train! It saves time (you gain one day) and especially money. But sometimes unable to get a empty couchette and there is always the risk of theft of backpacks (not very common but ...), however there is always possible to discretely tie the bags in the racks of the couchette; objects to make noise are also welcome! If you want to sleep in a sleeping bag, the advise is to bring the core values (money, documents, etc..) under your cloths (next to the chest, for example). For the nights in dormitories there are other methods such as putting the values within the pillow pad (though sometimes the cleaning ladies take the pillow cases) or inside of the sleeping bag. It is important to remember that if someone loses the documents and the Interrail ticket the journey ends there! Therefore, please be aware.
Options that are totally free as sleeping on trains, at train stations, underneath the bridge and the most viable of all ... at home of acquaintances!