I had heard about the baby powder idea but thought it was just someone's crazy idea. Now I know it really works!
CAUTION - Before you slather on a lot of oil, know your saddle. If it's a custom made by a saddler worth their salt, they will have oiled the saddle appropriately during the construction process. Depending on who built it, and how it was tooled, they may have applied a finish such as Tan-Kote to protect it in addition to oil so if you try to oil the grain side, you might just have an oily mess. Talk to them to find out about the finish and care. Also, make sure you use pure neatsfoot oil, and not compound. You can also use olive oil. After letting the oil soak in overnight, you can apply leather balm with atom wax to the back of fenders and stirrup leathers to help prevent sweat and dirt from being absorbed into the leather. Only use it on the back though. After you apply it, use a piece of very faded denim and rub it down. Some of the squeek can come from new leather as it stretches and flexes. Personally I would avoid baby powder, or any powder since it will tend to absorb oils from the leather. When properly constructed the ground seat will not move, nor will the leather seat. If it's moving and squeeking, the saddle is of poor construction.