Not washing it is probably the best bet for longevity. Hose it off and hang dry it out of the sun.
Washing waterproof fabrics degrades the DWR (durable water repellant) finish. With a waterproof membrane fabric like this bivy it won't be water permeable without the DWR but it will effect breathability as the outer layer of fabric protecting the inner membrane will saturate with water not allowing the moisture from inside (i.e. your sweat) the bivy to escape and you'll wake up wet from your own moisture.
If you have washed the bivy with detergent or it really needs a more thorough cleaning you can retreat the outer DWR coating with a DWR treatment product found at most outdoor stores.