Most Helpful Guy
Try hand training.
Chihuahuas are very smart, so you've got a decent chance with them. I've tried training my gfs' little dogs and it isn't always easy. One time I was mauled by a whole pack of Pomeranians because I must have offended them with one of my hand gestures. Don't throw a west side sign to some east coast lap dogs. Those Pomies don't play.
Here's a good site that talks about an intuitive way to get most dogs to obey, because it comes natural to them to follow your hand movements: link
As far as the aggression goes, Caesar whats-his-name from the Dog Whisperer is right: make a loud noise - usually a hiss works great, and clap your hands at the same time, or lightly touch them with your foot if you're walking them. Don't punt them. Chihuahuas have great hang time, but they never stick the landing. Just tap the hindquarters or even stomp the ground a little next to him. After several of these, you can nix the stomping and clapping, etc. The effect of the hiss will be the same.
I always take my dogs to the dog park, too. If you have one in your area, it's great to get them socialized and used to all the sensory input. Just watch out for Pomeranian gangs.