Now this one is definitely a plug for Peggy Halls book…. if you have not got a copy, get one. Trust me.
One of the best things I learnt about keeping marine heads clean and operating was icecubes and laundry detergent.
You add them both to your holding tank, just before going to a, preferably bumpy upwind sail. The ice cube of course will eventually melt and leave nothing behind, but until they do they are great little rocks that smash about knocking what you want out of your holding tank off the sides.