Do not allow leather to come into contact with the chemicals of "spot-on" flea treatment. This will damage the leather.
If leather gets wet, allow to air-dry completely before re-use. Do not dry on a radiator.
To clean leather, use saddle soap. Leather will also benefit from restoratives made with beeswax.
Metal fittings can be polished with Brasso or similar.
Keep all chemicals away from dogs.