I practice with an empty syringe and a pocketful of treats ahead of time, simple approach and retreat exercise. I put the syringe as close to the horse's mouth as he's willing to let me, and I hold it there until he's relaxed and not caring, and then take it away and praise. When I get it into his mouth, I give a treat and take the syringe away and praise. Usually the worst I have to deal with on D-Day is 1 or 2 approaches and retreats when they smell the medication. I always treat after I deworm to "make peace." If they're really a lot more fussy during the actual deworming I'll practice with a syringe loaded with applesauce. I also try to find the active ingredient that I need at the time in an apple flavored version. My guys seem to handle that better; I get fewer sour looks afterward.
I have really small hands, and it's hard for me to hold the tube and get the plunger down with one hand, so it's important to me to be able to take my time and not have to fight the horse to do any oral medication. My vet appreciates it, too.