If the horse is going to fast in the walk, try to sit deeper and NOT follow the motion of the walk. As she steps each stride, don't let your pelvis flex to follow her stride. Suck your stomach in and hold deep in the saddle. This blocks her ability to take long strides, making the rhythm of the walk slower. Like mbender said, you should also try to keep her mind busy. If she is going to fast, get her to do circles, bending, etc. If you are out on the trails, ask for shoulder in, slight leg yeilding, etc. Not really strenous, but enough to keep her mind occupied.