A good way to prevent damage from tie downs is to have some padding on your racks, this helps secure the board better without having to tighten the straps too much.
Also maybe consider the straps you use, the wider the better to spread the load.
I find Puka patches a great quick fix to keep you on the water. As soon as you notice a crack or chip just stick one on. They go on wet or dry and are instant. Worry about a permanent repair at a later date.
Check out the Puka Patches here -
www.briskites.com.au/index.php?l=product_detail&p=1255