As a few have mentioned:
Tuck the head under wing before freezing and place in a plastic bag. Smaller ducks will fit in gallon freezer bags. Tucking the head prevents breaking the neck in your freezer or the taxidermist's.
Don't put the duck in a plastic bag until it goes on ice or in a freezer. This will delay cooling and may cause feathers to slip. Don't put duck in a stocking. This also delays cooling and can crease some of the feathers (especially true with net type stockings).
Ducks can stay on ice a couple days and be fine for mounting.
If freezing for any length of time, a chest style freezer (non frost free) is best; this significantly cuts down on freezer burn issues.
Early season ducks are tough. If you are looking to mount a woodie, look it over good. The can be lots of pin feathers on early season birds.
Lastly, find a good bird taxidermist. Some guys can make ducks look alive; not all taxidermists can pull off a good duck mount.