Check the ticks that are present in your area so that you can readily identify them.
Perform tick checks of your horse which involve looking at the chest area, between the legs, under the chin, between the buttocks and under the tail.
Pasture maintenance can also help to some extent, as by keeping pastures mowed you reduce the number of ticks present. Ticks need humidity to survive, and long grass provides a much more humid environment than short grass. You can also, for the same reasons therefore, mow around the fence line of pastures, to reduce the length of the grass in the surrounds. The Center for Disease Control in America recommend that a three foot barrier is provided around the edges of paddocks, and actually around areas where human’s work, to reduce the likelihood of tick contact. Removal of dead logs or stone walls from paddocks and surrounding areas can also help as these tend to house rodents who are frequent tick carriers. Essentially, anything you can do in the pasture to open it open more to the sun and reduce the humidity will be of benefit, as this makes survival for ticks more difficult.
Repellants are also much more effective for ticks than they are for flies, as ticks have not developed the same levels of resistance as the flies to the chemicals. Sprays contained pyrethroids, or spot treatment with pyrethroids, can actually be very effective against ticks, as can fly boots that are impregnated with pyrethroids.