Picking your dog up is not going to stop an aggressive dog. He will just attack you, too. If a dog comes up behind you, there may not be time. One shake of a tiny Yorkie's neck can kill it.
IMO, everyone should carry doggie mace (Muzzle). If you live in an area where very aggressive breeds like pits may be lose, carry bear repellent.
Amazon.com: Mace Brand Muzzle Dog Repellent...