Tea tree oil is really nice. It repels several types of insects, not harsh on chemicals, and it detangles the mane and tail nice. You can get straight oil, or use shampoo infused - just when using the shampoo, don't wash it out completely.
I just use a tick remover to yank them off, but have heard that vaseline, nail polish are good! :)
Cooper's Spot On, 10ml dose for horses and 5ml for ponies, dribbled along the spine from withers to tail, it's systemic and the ticks drop off in a couple of days, it also helps keep flies away, brilliant!