Wipe them down, don't wash them. Minimal leather sneakers and leather sandals both clean up with a damp cloth and a little mild soap, not a trip through the machine — water and heat warp the soles and dry the leather out. Wipe the uppers after a wear, especially on white leather where every mark shows, and let them air dry naturally.
Feed the leather now and then. Even casual leather stays supple and crack-free with an occasional thin coat of a neutral cream or conditioner, buffed in and left to sink. Do it sparingly — over-conditioning softens and darkens the leather — and pay attention to the flex points where sneakers crease and where the sandal straps bend.
Mind the footbed and the sole. Sandals worn barefoot soak up oils and sweat in the footbed; wipe it down and let it dry fully between wears, and a smear of leather balm keeps it from going hard. On the sneakers, a soft brush clears grit from the welt and cup sole before it grinds in and marks them.
Rotate and dry slowly. Give any leather footwear a day off to dry out and hold its shape — trees or loose paper help — and never dry a soaked pair by a radiator, which stiffens and cracks the leather. Rotated and wiped down, casual leather takes on a good patina instead of wearing out.