Boulevard de Waterloo is a great street for shopping on both sides, they have little boutiques to main stream designers. It is perpendicular to Ave. Louis, the part close to B. de Waterloo has nice shops too.
Antoine Dansaertsstraat is also popular and trendy.
If you want some more Belgian style or unique boutiques, then Antwerp is better. Walk out of the Central station, then walk straight down to Meir, especially towards the end of it, that's where the little boutiques are tagged into the side streets.
Great shoe shopping in Belgium.