Do you have a real bullwhip? If so , try this:
Theres a few ways you can 'fake' your shots. These rules apply to fighting, and basically hitting anyone with anything. Me and my friend will do these types of things in public places, to see what reactions people have. Great fun
Place your camera behind your actor, and extend your whip. Keep your actor about a foot and a half away from the maximum distance the whip can travel. Whip the whip, and have your actor pretend like he just got hit in the face or what not. When you playback the video, it will look exactly like he just got whipped for real. Cameras tend to lose depth perception, so this technique works really well.
You could also try standing in front of the camera, parallel to your actor. But instead of lining up with him, stay about a foot to the left (back from the camera). Once again, due to no depth perception, it will look like you two are lined up, and you can whip away freely. Hope this helps.