What, if anything, determines a survivor's success at fighting a Jockey's attempts to move them in a direction?
One thing I do as a survivor that seems to help is to repeatedly tap my directional keys in the opposite direction the Jockey is trying to move me. This appears to be more effective than just holding the key down continuously.
Here's some advice from the L4D wiki:
I tend to run up and melee my teammates with a Jockey on their head and then shoot it, as it's usually a quicker way to save their health bar than just shooting, especially since the update where they increased the Jockey's DPS in Versus.