Answer

By Your Horse

Saddler Andrew Reilly advises:

Some ponies are shaped like smooth milk bottles, the narrowest part being just behind the elbows, and every saddle you try slides forward even when girthed tightly. Saddlers are presented with this problem all the time and there are several courses of action that can be tried to remedy the situation. The most obvious is to have point straps fitted to your saddle. This is where an additional set of girth straps are riveted to the foremost secure part of the saddle tree, known as the tree points. This results in the girth being anchored further forward on the saddle, so when the girth migrates forwards to the pony’s elbows, there is more chance of the saddle staying in position.
Cruppers can also be used to keep a saddle from slipping forward, although I advise against this if thehorse’s dock area is sensitive.
Other attempts to keep the saddle back have included using a damp chamois leather between saddle and pony, swapping your numnah for a ‘sticky’ item such as a limpet pad, or trying a numnah with thousands of tiny rubber- like nodules moulded into its surface. These methods all rely on increasing the friction between your saddle and pony.