What I've found to work the best is to balance out a negative with a positive. On a regular basis, mix up a yummy concoction of applesauce and honey, (I put about a tablespoon of honey in about 1/4 cup of applesauce), put it in a clean syringe and periodically give it to your horse just as a treat. You can give it to them once a week, a few times a week or once a month, depending on just how much the horse hates and avoids the tube. The more they hate it and fight you on it, the more often you give it to them in order to get them used to it. One good taste in their mouth and they rapidly get used to it!!
If they really fling their head around and won't let the tube near their mouth, all you have to do is get a drop of that yummy stuff in the corner of their mouth or on their lips (even if you have to put it on your finger a few times first) and let them get a taste of it.
Do this several times BEFORE its time for the dewormer. Once they realize that the tube more often means a tasty treat, it will be so-o-o much easier to deworm them when the time comes.
You can experiment with different mixtures based on what you have in the kitchen and what your horse likes best. Applesauce, oatmeal, honey, molasses, etc. (Just remember to be easy on the sugar and sweeteners. If you give them the syringe treat, you might want to cut down on the other treats you normally give).
This works really well with giving oral medications, too. I have 2 syrings in hand. I give one syringe of applesauce & honey, then the medication, followed by another syringe of applesauce. I knew about this trick for quite awhile but I really got to put it to the test with a yearling Thoroughbred filly I was treating once that had a bad, deep rooted case of pneumonia that just wouldn't go away. I had to give her oral antibiotics for 2 solid weeks. After the very first dose of meds, I knew I had to get creative in order to get us both through the next 14 days AND to keep her from developing a strong aversion to the oral syringe. She loved it!!! She actually got excited about her applesauce "cocktail". :D She nickered for it and sucked it up so quickly that I could barely get it in her mouth fast enough. She barely noticed the dose of medication in the middle at all. What a great lesson that little filly taught me.