Background:  I'm starting my own clothing line and I'm trying to start
a buzz.  I want to send out a postcard to the people making the
fashion decisions for magazines and department stores.

Question:  How do I reach the people making the fashion decisions for
magazines (Vibe, Ebony, Spin, Essence, XXL etc.) and Department Stores
(Macy's, Bloomingdales, Saks, Footlocker etc.)

You probably want to start with a certain own personal email like
hotmail and type in a city under long range and it will send whatever
you want to every body!
The trickey part is is to not make it go to there junk mail account
thats bad
You want to make it so there is and important sign on the side when
you send it.
If you cant make the exclamation mark then try a different email like
yahoo
or hack on to a popular web site and post your message there

For any magazine, you want to send information to the publisher of the
magazine, to the attention of a specific magazine, and if you want to
address it to a specific writer in the magazine who you think will be
interested in your product, then address it to her/him.  You can find
this information on any magazine website by selecting "About Us."

Ebony/Jet is owned by Johnson Publishing:

http://www.johnsonpublishingcompany.com/

They have a "contact us" page and you also might be interested in the
Ebony Fashion Fair:
http://www.ebonyfashionfair.com/
You might want to see if they are looking for volunteers to work at
the fair when it comes to your city, then you can learn more about
getting into the fair.

Spin is published by Vibe/Spin Ventures and their address is:

Vibe/Spin Ventures 
215 Lexington Avenue
6th Floor
New York, NY 10016
info@vibe.com

Essence is published by ESSENCE Communications Partners:

1500 Broadway
6th flr, New York
NY 10036
212-642-0600.  
http://www.essence.com/essence/aboutus/0,16109,,00.html          

XXL is published by Harris Publications, Inc:

1115 Broadway
New York, NY 10010
212-807-1479

You may also want to send material to Conde Nast Publications since
they publish a million magazines:

4 Times Square
New York, NY 10036
http://www.condenet.com/condenast/

Or they also have locations in Los Angeles and Chicago.

And a little advice:  "buzz" is a pretty nonspecific and subjective
term.  If you want to start in magazines, you should start with
smaller, independent fashion magzines.  Major magazines only endorse
designers that buy up advertizing.

Be aware that to get into a department store you will have to invest
quite a lot of money to be able to meet their production demands. 
They don't give you money up front.  Department stores are large
enough, have a wide enough audience, invest enough money, and deal
with major enough designers that they rent out their floor space,
catalog space to what products would be most profitable for them. 
Department stores and magazines are all about marketing.

How do you reach these people?  When starting out as a designer you
have to invest in your product first; work on your line, then invest
in opening a boutique maybe with some other artists or designers. 
Once you know the design community then you can market your product
through events with other designers, artists, at galleries and fashion
shows.  Working your stuff into whatever scene you want to work into:
clubs, malls, music stores, talking it up to people.  Then maybe you
can get an agent to represent you, to negotiate deals with department
stores.  See once you have a line, anyone can wear it thus exposing it
and you to consumers and industry people.  A magazine wants to know
how your product will help them sell more magazines to consumers.

You can also buy a booth in a fashion trade show where stores look for
designers they want to market.  That is where most designers start in
order to make their way into stores:

http://www.apparelnews.net/TradeShows/

That's just one link, there was tons of information in a google search
for "fashion trade shows."

I hope that helps.