We started with the obvious option: Immoweb (http://immoweb.be/en/). Great website, available in English, allows you to enter tiny-winy details about the area, size, price, number of rooms etc. You can save your search criteria, subscribe to alerts and its all very handy.
To make the house-hunting process even more efficient, we have decided to sign a contract with Property Hunter (http://www.propertyhunter.be/), a young Belgian company, which is specialized in real estate searching (for individuals, for companies, for buying or for renting). They are super dynamic people (no wonder they've won the "best property finder" award in 2010!) and their contribution makes a big difference.
The idea is: you let them know the details of your dream place, and they help you find it. They are in contact with different real estate agencies, they are surveying much much more than what you would be able to do. Once they identify something, they visit it, take pictures and write you a feedback. You only visit yourself, if you are convinced by their reaction. Needless to say, this saves a lot time and effort!
Another reason why we found their services invaluable: they recommended someone to supervise our renovations. We knew we could trust this person, because he came with a good reference, and in retrospect I have to say that it would probably not have been possible without him.