Socialization
First and most importantly,  and I cannot stress this enough.  PLEASE
CRATE TRAIN YOUR PUPPY!
 Crates are NOT cruel.  They are simply
tools that provide a safe "den" type of environment for your dog and give you
the opportunity to train your puppy properly so that you will have a happier,
more well-adjusted adult dog.  Crate training is essential because that single
act teaches the puppy so many lessons.  A well crate trained dog recognizes
boundaries, is tolerant of being left alone, recognizes you as his leader,
understands the benefits and rewards of training, and is generally a more
pliable dog.  I have provided information on crate training in the material I
sent when you reserved your puppy, but you can also do a quick search of the
web to find hundreds of sites that explain how best to execute the process of
crate training.

Next, ALL dogs, including Chihuahua, and maybe even especially
Chihuahuas, NEED training.  Obedience training is a good start, but it is not
the end.  A good puppy kindergarten class is a superb idea for all puppies
and it is a great socialization opportunity.

Socialization is a form of training, and it desensitizes your puppy to common
stimuli that can cause fearful reactions later in life.  So, socialization is NOT
optional.  It is REQUIRED to have a well-adjusted Chihuahua.

Again, I cannot stress this enough!  In Chihuahua, perhaps more than in any
other breed, socialization is crucial to ensuring that your puppy is happy,
well-adjusted and well-behaved.  Many people have cited instances of
Chihuahuas being mean, unsocial and intolerable to strangers and other dogs,
and that characterization of this breed breaks my heart.  Chihuahuas are
raised to be, quite literally, man's best friend.  This breed is not a working
breed.  They don't herd or hunt.  They are bred to be companion animals, and
they do attach very strongly to their primary caregivers, but that does not
mean that they need to behave like living terrors to everyone else.  The key to
preventing that is thorough socialization

Our puppies are raised in our home, where we work from the day they are
born to socialize them.  The pups are handled and loved on every day,
usually several time a day, from the minute they are born until they leave us.   
They are introduced to other breeds whenever possible, new sights, sounds
and textures, and other people.  We do our best to give you a great start on
their socialization from the beginning.

However, this next sentence is the most crucial part of this page.  
Socialization does not stop here.  Because this is so important, I will say it
again.  SOCIALIZATION DOES NOT STOP HERE!  In fact, our efforts to
socialize your puppy are only the beginning of the process.  When your pup is
ready to leave, at about 8 weeks old, he/she is in the very beginning stages of
the critical socialization period, which begins around 8 weeks and ends
around 16 weeks of age.  That means that it will be largely up to you to
complete the socialization process for your puppy.  

During this critical period, it is essential that you introduce your puppy to as
many new things as possible.  You will want to introduce him/her to other
breeds of dog, other types of animals, many different types of sounds, floor
surfaces, scents, and especially other people.  You should begin with a
gradual introduction as your puppy settles in to your home, but once settled
in, you will want to get very serious about the process very quickly.  

The first step is to take your puppy with you everywhere that you possibly
can take him safely.  Let him get used to riding in the car, experiencing new
places, meeting new people and/or other dogs, and sniffing the world around
him in a calm and relaxed way.  If you wish to use specific training sessions
to enhance socialization, generally, short, calm and relaxed sessions are
best.  

For example, if you are introducing him to a new sound, keep it at around 2
minutes at first, and try to introduce the sound during a cuddle or play
session.  If the sound startles him, try using a sound app or website that
allows you to adjust the volume.  Start out softer and gradually get louder
until you can play the sound at about its normal volume without startling your
puppy (this works well for things like vacuum cleaners).  

We do our part while your puppy is with us, but no puppy is going to be a
ready made, perfect pet as soon as they go home.  Your puppy is still young
and still has much to learn.  So, you will still have training to do when your
puppy goes home with you. but with proper care and effort, your puppy will
be very social with other people and other dogs, and will be confident and
comfortable in new environments and situations.  I have included some links
below that offer more helpful tips and methods of socialization.  

Links:

Tips from Whole Dog Journal

Quick and Dirty Tips

Introducing to Dogs and Cats

Jumping and Barking

Moving to a New Home
Even well socialized pups and dogs can have some form of separation anxiety
when left alone, especially if they are NOT crate trained.  That is because
dogs are highly social animals. If you think about it, that should be intuitive.  
Dogs are pack animals.  They are instinctively programmed to live in groups.  
When a wild dog gets separated from his group, he will howl and cry to
facilitate reunion with his group.  Once reunited, the undesirable behavior will
stop.  If you have a single dog and no other animals, then YOU are his group,
and when you leave, he may howl or cry until you return and he is reunited
with you.  If that doesn't work, destructive behavior may ensue because your
dog cannot get out of the house, where he knows you went, to try and find you.  
This is separation anxiety.
 Arguably, the easiest fix is to crate train your
puppy from the start, which should eliminate the anxiety during separation.  
Another easy fix for separation anxiety is to provide your dog with an animal
companion so that when you leave, your dog's entire group hasn't left him
behind.  We often place our puppies in pairs, partly for this very reason.  
However, if that is not an option for you, then please read on.  
We start separation training while your puppy is with us, but keep in mind that
your puppy is still a puppy when he goes home.  He still has many phases to go
through and much to learn, so as with all forms of socialization, you will need
to continue his separation training.  The links below give you step by step
instructions for preventing and reversing separation anxiety.  Just click the
titles to read the full articles.

How To Deal with Puppy Separation Anxiety

Five Steps to Preventing Dog Separation Anxiety

Inappropriate Chewing and Separation Anxiety

Dealing with Dog Separation Anxiety - Cesar Millan