What type of sign and Training?

I've tried to do some reading about whether signs can increase/reduce liability and my conclusion was it's a crapshoot.  An attorney who wants to sue you can and will argue anything.  If your dog bites someone, and you don't have a sign warning them, then they will most likely argue that you should have warned them.  If you warn them, then they will try to say you knew the dog was dangerous.  Damned if you do, damned if you don't.   So beyond any special dog laws in your state (which should be checked first), I think the best thing to do is to somehow (in whatever way you are comfortable with) is to warn people that their is a dog behind the fence.  I think this is only fair to neighborhood kids, utility workers, delivery guys, etc.  I do plan on putting up a Beware of Dog sign. Like a stop sign, it's easily recognizable by even people who don't know dogs.  You'd be amazed how many people can't read or don't read signs.

In the city of Boston, there is actually a "Pitbull ordinance" that requires you put up Beware of Dog signs at your residence.  But basically in the state of Massachusetts, if your dog bites someone you are liable, unless it's determined the person was trespassing or harassing the dog.