We use Frontline Tritak on both of ours. The ticks were really bad this spring but have subsided some here in eastern NC. Of course we only let the dogs walk in either bush hogged areas or where I mow with a mower. As long as the grass is relatively short we don't have as much problems.
Frontline starts killing the ticks as soon as it bites the dog. Generally when I check them I find dead ticks. Normally the only live ticks I find are immediately after a walk.
I am not sure there is a "true" tick repellent for dogs on the market so we just keep them out of tall grass and the woods. Heart worms would be our biggest worry if not for heart guard pills.