I would be checking everything that could cause the bike to be running rich i.e. air filter, spark plugs, valves. Just start with the easier and free things first and work your way up the list. Also ask yourself if there is anything that you have changed on the bike when the problem started.
+1 on that.
Dirty, plugged air cleaner could cause the problem. How many miles since it was changed, same with the plugs and oil and filter. Also a dirty O2 sensor can cause issues. Good place to start.