Originally Posted by rockymountain
Looking for a shaped girth for my girl - she has a rather large tummy ( used to be a brood mare ) and saddle slips forward and side to side....
I suspect your saddle just isn't quite right. It looks cantle high and it might be too curved from front to back for your horse. Being too curved can make it lift at the back in rising trot and/or canter (what happens when you press on the pommel when the saddle is just placed on the horse's back?) but can also make it laterally unstable, as can the bars of the tree being too upright. These are the bits that run from the head at the front back under the twist and into the seat, and they need to be as close as possible to the angle of the back - the wider the horse the flatter those bars need to be. Most horses lift their backs in work and effectively fit "flatter" than their back looks at standstill.
I also find that some of these brands you're all talking about just sit up too high - you need a saddle that really wraps around the horse to take the centre of gravity down and around, and give better panel contact and therefore grip.