Bolsters allow you to change the shape of your Sensation's seat easily. There are a few different kinds to be familiar with:

Cantle Bolsters are a crescent shaped insert that fits between the seat and the cantle of the saddle. This shortens the seat a full size. The Cantle bolster is recommended for use especially when smaller riders are using the Sensation. $25

Pommel Bolsters are a triangle shaped insert that fits under the seat and up against the pommel. The pommel bolster gives a more sloping rise to the pommel, and can give the rider a feeling of added twist and a less dramatic rise to the pommel. The pommel bolster is especially popular in the Western or Westlish models, as it gives the rider the feeling of a "pocket" in the rear of the saddle, akin to many traditional western saddles. $20

Twist Bolsters are a diamond shape insert designed to fit under the seat along the center seam of the saddle, and just up to the pommel. They are useful to riders who have horses that are very wide, or who have troublesome hips and like to feel a more extreme twist (feeling of narrowness) in their saddle. $20