- keep them out of the sun.
- rinsing them is beneficial, but not mandatory. I must say I rinse them only from time to time (once every month?). Especially useful for the metal and straps/neoprene parts.
- I just store them loose in the bag with the board,
not wrapped around the tail
Bends:
- avoid tight bends
- avoid tension
- if you have bends, just put them in hot (boiling) water for 5 minutes, and let them cool stretched. Or just stretch them horizontally in the garden and pour slowly water from a kettle on them.
For storing spare leashes, I reproduce the system used by some leashes such as the O&E ONE XT: a cardboard square with 4 ties to keep the cord in a perfect circle without tension: This avoid bends and tangles and minimize storage space

Note that O&E actually do not use ties but have the cord loops hold in a cardboard box, which is even better:You only need to tie the cuff and railsaver parts and let the cord loose to avoid any bend-creating tensions on the cord.