bit late now for the OP but for anyone else stumbling over this thread my advice echo some of what mark said, get in a position that you can shoot a few jumps from.
put the camera into servo or continuous focusing (you dont say what kit you have so..) and pan for side on shots, also try and pick those without a cluttered background. a nice blurred background is also good so adjust your aperture accordingly depending on frontal or side on shots.
re spooking horses, jumping horses arent normally too bad (they normally get "focused" after a whack from the rider..). but generally if a rider decides youve spooked their horse whether you did or not youll get some flak no matter where youre stood.
also a word to the wise, jumpers are tighter than a ducks proverbial and very rarely pay for shots if they can help it. look out for people taking the images off of your website if applicable.