You need to establish a cue for forward motion before you ever get on, whether it is a smooch, a cluck, or a slight tap on the rump. Don't get to confident that she isn't going to do anything based on one ride. I have ridden many young horses that didn't throw their first buck until a couple of weeks under saddle. I would also suggest that you always keep the inside rein in light contact with her mouth and if she starts to buck, then pull that one rein just enough to keep her bent because that will take a lot of her power away. Do you have her soft to the bit each way?