Assuming it's just fouling (and not melting :)
plug needs to be gapped correctly, installed and torqued properly.
ignition has to be in good shape.. capable of delivering a strong spark. Ignition wires, plug wire, points and timing, etc.. everything's gotta be working right.
Too much oil in the fuel mix can foul plugs. Does the bike smoke a lot? oil dripping from the tail pipe?
Running too rich due to carb problems can foul plugs. A perpetually, partially closed choke might do it too.
so, first tune the bike up .. if everything else is working properly and the problem persists, get a hotter plug.
NGK B5HS is hotter than the B6HS. (NGK B7HS is colder)
The plug heat range should be hot enough to keep the plug clean when everything esle is in tune... heat range will not normally affect how the engine otherwise performs.