I have used the suggestion about practicing worming with something yummy, with great success. I like to use custard powder with some water to make a paste, and use an old worming tube to dispense it. I'll do it weekly on a difficult horse, until it becomes routine. Then when the real wormer comes out, if you're quick about it by the time it's registered that the stuff tastes bad, its all down their throat.
The other tip I have, is to not make a big deal of worming. A lot of people will fuss about, halter the horse, take it to a big open space, fuss with getting the lead rope in just the right spot, pat the horse, tell it how good it is, quietly try to get the wormer...... you get the idea. Horse realised 10minutes ago that you're going to do something unpleasant.
I just stick the tube in my pocket, put the halter on like I'm about to take him out to the paddock, and super quickly just whip the tube out, put it into the corner of the mouth, and squeeze. Over and done with in seconds, horse doesn't know what hit it.