Namur (French, in Dutch: Namen) is the capital of Wallonia, the French speaking part (southern) of Belgium, situated on the E41 and E42 motorway. Namur is both the capital of the province of Namur and of Wallonia. It stands at the confluence of the Sambre and Meuse rivers.
North towards Brussels, Louvain-la-Neuve and East towards Liège
If you want to go to Louvain-la-Neuve or Brussels the best place to stand is at the entrance of the highway. For this, you should go to the central train station of Namur and take the bus n°27 (every 20 minutes +/-) toward "Bouge" and "Champion". This bus is across the street of the station, on your right. In front of "La Maison du TEC". You ask the bus driver to take you until the entrance of the highway of Champion ("Sortie de Champion). It will cost you EUR 1.70 and it takes about 10 minutes.
You can as well hitchhike to get there, leave the station by the back exit, turn right and walk 100 meter until you reach a bridge on wich there is bus stop were cars can stop easily. You might get very fast a lift to the highway entrance or even the your final destination.
From the bus stop walk 100 meters in the same direction of the bus (as you follow the bus). You will soon see the big green signs of the highway E41 and E42 You can stand either on the beginning of the entrance or on the in the middle of the brige where cars have stop to turn right becasue if the oncoming traffic, in the rush hours, line forms there and you can have the opportunity to talk to the drivers.
South towards Luxembourg
Take the same direction as indicate already in the top. You should hitchhike from the entrance to Luxembourg.
There is a petrol station in Belgrade (the name of that part of the city), you can stand in front where cars can stop. Cars can be slow down here because of a traffic-light. Is might not be a perfect stop but if you're lucky you can be picked up. You can take bus direction "Belgrade" or "Flawinne" that you can take in the central station. Ask the bus driver and step out before it goes off the big road, several 100 meters before the gas-station.
Another option is to go to the same spot wich is mentionned above for north and south direction. You might need a sign because drivers will be headed to many different destinations from there.