Answer: Based on your description I am not certain what is wrong with your holly so it is difficult to make a recommendation.
Hollies naturally drop their oldest leaves in the spring so some of what you are seeing may be normal.
Winter damage can also cause foliage loss, either due to excessive cold or fluctuating spring temperatures or excessive wind exposure. If the wood is still alive, new leaves will grow and replace those that have been lost. You could trim off any dead wood beginning at the branch tips. Live wood will be supple and have green inside the bark, dead wood will be brittle and snap in your fingers and will be off-color dull gray or brown. Remove any dead wood and then if necessary trim a bit more to get a somewhat symmetrical look.
Another cause for sparse thin growth is lack of sunlight. Hollies prefer full sun but will tolerate some shade. In a shadier site, they will grow less dense. There is really nothing you can do to prevent that.
I hope this helps.
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