BioThane® is easier to look after then leather. Majority of the time it only requires a wipe over with a wet cloth. If very dirty leave it to soak for a while in hot soapy water.
Make sure you check your stitching regularly. For example, If your horse pulls back or treads on the reins, the stitching is designed to break.
Little Scribbles will repair any items for just the cost of any new materials used and return postage.
Stainless steel doesn't need any looking after apart from the clips and hooks.
Brass and nickle will come up like new with a little metal polish. Please note though if you do ride your horses a lot through the sea, salt water does erode metal over time.