Cut the plums in half and then cook with the stones in. They will float to the top and you can skim them off without any hassle.
Be generous with the spices - cloves, star anise and black peppercorns, even a few chilli flakes, all go well with plums. Put whole spices in and then fish them out at the end. Brown sugar is better than white.
I've got another 'keen'... Ouch!