To echo Jay's comments, keeping leather waterproof and supple isn't hard, but leather needs regular care and attention.
Leather is skin, and like all skin it is essential to keep it clean and moisturise regularly. Monthly would be good in the summer, less in winter. Go by the feel, it should feel "mildly moist"! If SWIMBO rides a horse ask her...involve her!
Don't buy the expensive stuff supplied by car polish companies, go to your local equine supplier and by leather cleaner intended for saddles, bridles and other tack.
We use this on all three cars:
Leather Cleaner An added bonus: it leaves your hands all soft and smooth!
