North-West towards Brussels / South towards Namur, Luxembourg, France
From the city center walk all the way up to the roundabout of the shopping center "Esplanade" (about 10 min). While walking along the Boulevard de Wallonie, you'll see a petrol station, but you better walk a little longer and stand on the roundabout behind the sign "E411 Bruxelles". It is THE perfect enlightened spot to go either north or south (road splits a little further). You may definitely meet other hitchhikers...
If you are traveling south, and you don't want to stay in Namur but rather keep going south, ask the driver to drop you at the roundabout of Namur centre exit. Same kind of spot...
Stand at the beginning of the RN25 (15-20 min walk). The spot is not that great, but you should be fine with a sign !
Hostels/ Hotels : There is a inn near the city-center which is called "Le relais"and a quit fancy hotel: the "hotel mercure". Don't forget about Couchsurfing ! You can always walk 20 min and find some bush to sleep in outside the city.