Make use of the Housing Service. Why?
1) They will inform you about renting in Leuven, give you some warnings beforehand
2) They have a database www.kotwijs.be . The landlords at this website are checked out by the Housing Service. Students can come to complain about landlords here. If the landlord doesn't fit the kotwijs standard anymore, he'll be crossed out of the database. This makes it the safest database in Leuven. You can consider it to be a "white list".
3) At the Housing Service, you can make use of telephones and computers. Free of charge. You will also get some advice and assistance when you have questions about the offers on Kotwijs.
4) If you found a place, you should pass by the Housing Service for a translation of your contract. It is the only way to know you are not signing a bad contract.(For legal reasons, contracts should be signed in Dutch. Even if the landlord offers a translation, it is best to go to the Housing Service for a check up before you sign)
5) Make sure to check out: http://www.kuleuven.be/english/studentservices/accommodation
6) If you still can’t find a place to stay, you can try www.opkotinleuven.be ,an other site (not from KU Leuven itself), where you can find some places.