I never wash mine. After cooking, i leave the flames still burning (it's a gas one) for 10 minutes with the top shut which burns off the excess crud on the grills. It's then very easy to just run a wire brush up the grill to remove the dried black bits when it's cooled down a bit.
We used to have the same problem, until I discovered 'Tesco barbecue cleaner', Just spray it on with the gun,let it soak and do its stuff then wash it off as normal, grease etc washes off really easy. Good luck. T bird+
Ps, its pretty potent so wear some rubber gloves.