I always make sure there is at least some inkling of a cue for forward motion instilled in the horse before I ever get on; whether it be a word cue, smooching, or clucking. When I actually get into the saddle, I use that cue in conjunction with leg pressure and each time I have to ask and they don't respond, I increase the pressure. If the horse continues to refuse to move, I will bend their neck to one side and give them a firm pop on the rump with a bridle rein on the other. However, that is kindof a more reckless way to do it and could very well result in either a bolt or a buck. If you have someone who could urge from the ground while you ride in a roundpen, that would work best.