I give the leading edge a bit of a wipe with a damp chamois when packing up. It gets off any mud stains, sand or dust and minor abrasion marks from the sand.
Other than that, just pack it carefully and put it in the bag.
From time to time fly it down and land it on the water so that excess salt and dust gets rinsed off.
The biggest cause of damage is dragging the kite across hard and sharp objects like footpath and rocks and shells and bushes and stuff. The solution is simple ... don't do those things.
There's not a lot of point in rubbing moisturiser and cleansing lotions
on your kite. They last for several years easily with no real maintenance and lots of use. By that time there's a bunch of better new kites on the market.
No matter how good the condition of your kites they are going to depreciate at 50-60% each year.
.. 
