General maintenance is fairly minimal if carried out regularly and the pond need not be emptied every year, unless it has been neglected.
After a number of years, however, silt and debris will gradually accumulate on the floor of the pond, which may lead to pollution, causing both the plants and the fish to suffer. Leaks may also occur as a result of frost damage, ground subsidence or even deterioration through age, and these must be repaired as soon as they are spotted.
There are many little jobs to do - especially in the spring. Check your water pump is clean and working - you will start to need it now as the weather warm up.
If you really want to make life easy, it is possible to buy a pond vacuum cleaner which will clean the silt and debris from the bottom of the pool. It is also possible to do this with a bit of ingenuity and a stirrup pump!
Check the fish out to make sure that there are no fungal or other diseases accumulated in the lethargic winter months, when you may not have seen the fish at close quarters.