A few days before you plan to worm, fill an empty syringe (I save a few old worming or Bute tubes for this purpose) with applesauce or molasses and squirt that into your horse's mouth. Most horses love the taste. It won't take long before your horse will start reaching out asking for you to squirt things in their mouth. Give the wormer, then follow up with the applesauce for another couple days. They will think the wormer was just a bad batch of applesauce!
This also works great if you ever need to give your horse Bute. Hopefully you will have done the applesauce thing at some point prior to your horse's injury, so that they will be relaxed when you approach with the Bute. If you anticipate multiple days of dosing with Bute, you could go out with applesauce between Bute doses to help keep your horse guessing, or give some applesauce prior to the Bute. Mix things up a bit to keep your horse from anticipating you and you'll find that medicating your horse will no longer be a dreaded chore.