Customs and habits in Belgium are very similar to most Western countries. Below are a few that are generally observed:
Greetings – Belgians typically kiss to greet, either twice (on either cheek, largely in Flanders) or three times (one cheek, next cheek, first cheek in Wallonia/Brussels). Men shake hands, though often they may do the cheek kisses as well with friends. It can be confusing trying to work out how many times to kiss the cheeks, especially when you first meet someone, so the default would be three times, then when more familiar, kiss just once on one cheek when you meet again.
Party/Dinner Invitations – When invited to a party, dinner or an event, ask whether you can bring friends in advance as these are usually catered for by portions. It is also customary to bring a bottle of wine, flowers, chocolate or some small gift for the host.
Fries – “Good” Belgians eat fries at least once a week, “really good” Belgians eat potatoes (in one form or another) at least once a day. Fries are often eaten with mayonnaise.
Eye Contact – When having drinks, it is customary to offer “cheers” by looking the person/people in the eyes when clinking glasses.
Topics to Avoid – Unless extremely familiar with someone, it is generally considered bad form to talk about salaries/wages or Belgian politics (in particular the Flemish/Wallonian issues).
Table Manners – Generally good table manners (eating with a closed mouth, elbows off tables and so on) during the meal without being too loud and raucous apply, in particular with more mature company.
Dress Codes – In most places jean-casual is acceptable, though in some more upper-class dining establishments, a jacket may be required, and some night clubs require smart dress as well. Make sure to call ahead if you have any doubt.