wideguy

I've always loosened the top yoke nut with the bike on the side stand. Assuming you have a 6 point socket or spanner, turn it against the steering stop, put pressure on the wrench, and rap it good with a plastic or hard rubber mallet. Wearing a heavy glove on the hand holding the wrench is often a good idea.
If this doesn't work, try heating the nut with a heat gun or propane torch.