Move the Eglu to a new area of grass. This will prevent one area of your lawn being over-used and any disease from building up in the area.
Empty their bathing water and clean the container. You may want to do this a couple of times a week depending on how many ducks you have and how quickly the water is soiled. Fresh water won't be crystal clear for long because the ducks will bring mud with them and soon turn the water brown but if they have another drinking source that is replaced daily this isn't a problem.
Try to check that your birds are well every week by picking them up and checking for all the signs of a healthy duck as outlined in the Duck Health section.
Make sure that the straw, hay or shavings in the nesting box are clean and fresh.