Take the time to closely inspect your leash before going out especially on big/crowded days; look for nicks and scrapes
Wet&Dry the edges of your fins [400gt]
Weld broken leashes together; cut both ends back to the good stuff with a craft knife, melt the ends on an old iron [or a good one with a bit of greaseproof to protect the surface], press the squigey ends together hard and keep the pressureon for a couple of minutes, leave to cool for 30 min, trim it neat [or don't, it might be stronger]
Keep a spare in the car
Don't buy skinny little leashes, buy the longboard/big wave ones
Don't abandon your board when faced with big closeouts
Surf without a leash [good training for the last point]