For routine maintenance I always recommend neutral shoe polish, lightly applied to exterior surfaces, buffed with a soft cloth.

Basically just an inert wax, provides some protection against moisture and surface abrasion without saturating the leather. The neutral polish has no dye content so it won't stain clothing. If you have minor scratches or abrasions to deal with those can be touched up with leather dye (or a felt-tip of the proper color), then apply the neutral shoe polish.

Oily or greasy preparations are to be avoided because over time they will saturate the leather, softening it and overcoming the form-fitting qualities.