Asking for the right lead, for example:
* Sit slightly more heavily on your left seat bone
* Press your left leg straight into the horse's side just behind the girth
* Keeping your right leg at the girth, squeeze strongly with both legs to urge the horse foward.
*Maintain slightly more contact on the right rein as you lift it a fraction to riase your horse's head and tip his nose very slightly toward the right.
That's the standard aids. I have a ton more weird ones if none of those work.