Nami clearly has no way of aiming her lightning since all she really does is make a lightning cloud, so how does she never get hit especially since she's always holding a long metal stick.
This is largely opinion-based, but the answer is most likely control. Nami is an excellent navigator to begin with, she's used to her own attacks, and she's always careful to let lightning fall exactly where she needs and steer clear of it. After the timeskip, she was even shown to be able to aim where her lightning falls in a few attacks.
In contrast, on a more comical note, she almost did strike herself down with lightning by accident in the Enies Lobby arc due to her weapon being too powerful (And whacked Usopp/Sogeking over the head for it), but she quickly adapted to it.
As for the "metal stick" part, it was never adressed so only speculation can be used, but Usopp most likely designed it so as to not conduct lightning. Making a weapon that can create thunder but keeping it as standard metal would be heavily impractical and dangerous.