We had quite a few ticks last year so I smeared them with vaseline which suffocated them and they dropped off. For prevention rub power phaser fly spray/gel over the legs, chin, head etc and that seems to discourage the little pests.
We have a special pair of tic tweezers that have a light on them. They are meant for dogs but work just as effectively on the horses.
Failing that a hot burnt match usually does the trick just be careful not to touch the horse.