This is a huge pet peeve of mine since I've had similiar problems and posted here about it a bit ago. I can't find the original post but some of the suggestions I was given included:
1)Stepping in front of your dog and telling the other dog a firm no or go home. (I've found this works probably about 70% of the time) If nothing else it usually makes the other dog pause long enough for you to either to get out of their "territory" or the owner to regain control.
2)Carry a squirt bottle or water gun and spray the dog with water. Some owners get upset, but my feeling is they shouldn't have their dogs loose.
3)Be on alert. If it's a constant problem talk to the owners, explain the problem and ask if they could keep their dog confined to their yard. If you see a loose dog go a different way. I'm usually aware which houses have dogs and which have been problems in the past so I'm always alert to where that dog is and if there's anyway they can get out, open gate, etc.

I usually try to walk my dogs on the same day. I've talked to a couple of the owners, explained the problem (although most of them knew since they'd run out after their dogs) and told them which days I normally walk my dogs. Most have been really good about keeping them confined at least on those days.