I've used Obenauf's leather preservative with some success. It restores the dried-out leather but doesn't render it soft and mushy.
Here's a link to part of their site, dealing with horse tack. While reins and such need to be supple, saddles need to retain dimensional integrity. This stuff seems to work well for both applications. It was also recommended to me for such items as pistol holsters, which need to retain their shape to properly function.