This is the electronic gizmo that dogs don't like,

http://www.dazer.com/dog-deterrent.jsp

I have used one and it works on most dogs, but if you are unlucky enough to come across an aggressive, deaf dog this thing will not do much.

Haven't heard of using citronella, but i know a few motorcycle riders that use a squirt gun with ammonia. Have a friend that swears that his nieghbors dog always liked to chase him on his bike (he lives around farm land, so country dogs that like to chase things) until the dog got a shot right in the face. Now the dog hides under the porch when he hears a motorcycle.