They get greasy really quickly (and when oily drops start falling on the hob below, you know it’s time for action).
Most can go in the dishwasher on a hot wash, but if you don’t have one give them a good soak overnight in a hot strong washing soda solution – about a cup of crystals to 600ml hot water.
If they’re too big for the sink, stick them in the bath (line with a towel to stop scratches). If they’re still a bit greasy renew the solution and get the last of the gunk off with a nylon brush.