I'm surprised no-one has said this yet - piss on it. I keep an old plastic jug beside the toilet and once in a while I dose the compost bin with wee. The nitrates in it are like baby food for bacteria, instant nutrition that enables them to multiply faster. It does roughly the same job as the "compost activator" the garden centre would like to sell you. But you must add it when it's fresh. If it's left to stand, the nitrates turn to ammonia which is no use to bacteria or plants.