Stop, put your bike between you and the dog (and be prepared to use it as a weapon if necessary), square off your shoulders and "get big", and yell (anything "go home", "no", "bad dog", etc), maybe take a step or two towards them. Most will run back home before you get to the end of that sequence.
Try this thread as well for more ideas:
http://forums.teamestrogen.com/showt...highlight=dogs