I bought my ducks about 10 weeks ago and was told that worming was best to be preventive rather than reactive, in other words do it regularly to prevent the ducks getting worms and becoming ill rather than trying to treat them after they already have them. So I dutifully bought some verm-x worming pellets (its reasonably priced on ebay) which you get a small measuring spoon with. One scoop per bird mixed in with their normal pellets for 3 consecutive days every month. I believe there is a liquid form too where you can add it to their drinking water.
However, the site where I bought my ducks from has now issued some new advice where they are now saying that worming should only be done if the birds have worms. This is to prevent the worms becoming resistant to worming medication. So now I am confused as to what is best but I think I might just continue until I run of product. Here is the article from the website where I bought my ducks:
http://www.happychicks.co.uk/mall/p.../_eggsamine/226484/Eggsamine - Worm Egg Count