From the Galaxy Map Discussion thread...

List of some of the published Star Wars galaxy maps
Dark Horse Comic-Con exclusive map/poster (2006)
Star Wars: Empire at War (2006)
Star Wars: Complete Locations -- Inside the Worlds of Episode I to VI (2005)
The New Essential Chronology (2005)
Star Wars: The Ultimate Visual Guide (2005)
Risk Star Wars Clone Wars Edition (2005)
Knights of the Old Republic II: The Sith Lords (2004)
Official Star Wars Fact File (2000-2004)
Star Wars Insider #65 (2003)
New Jedi Order novels (1999-2003)
Cartoon Network Clone Wars website (2003)
Jedi Knight: Jedi Academy (2003)
Knights of the Old Republic (2003)
Inside the Worlds of Attack of the Clones (2002)
Star Wars: The Clone Wars (2002) (mission selection screen)
Star Wars Roleplaying Game Revised Core Rulebook (2002)
New Jedi Order Sourcebook (2002)
Star Wars Gamer magazine #5 (2001)
Inside the Worlds of Star Wars Episode I (2000)
Behind the Magic (1998)
Rebellion (1998)
Walt Disney Star Tours attraction (1987)

Regions
The entire galaxy is divided up into regions. These are the largest divisional groupings of the galaxy. They include: the Deep Core, the Core, the Colonies, the Inner Rim, the Expansion Region, the Outer Rim, the Unknown regions and Wild Space.

Sectors
A sector is a group or cluster of star systems united for economic and political reasons. They can be composed of anywhere from 50 to 1000s of systems.

Systems
Systems refer to individual stars and their orbiting planets. So the solar system in which we live is a system, not a sector. Earth is a planet within that system.

Heading
My internal method for tracking planets on the map. It's the direction from Coruscant where a planet can be found, assuming that Tatooine is always at a 180 degree heading from Coruscant. Headings are rounded to the nearest 45 degree.

Yavin 4: Outer Rim 090
Kashyyyk: Mid Rim 135
Naboo: Mid Rim 180
Geonosis: Outer Rim 180
Kamino: Wild Space 180
Bespin, Hoth, Endor, Utapau, Mustafar: Outer Rim 225

Wild Space
The galaxy's true frontier, Wild Space is the area once considered part of the Unknown Regions which was opened to exploration and settlement as one of the Emperor's last acts. Grand Admiral Thrawn was charged with taming this wilderness, and he declared it part of the Empire. On the maps included with the New Jedi Order novels, Wild Space is on the edge of the Rim of the known galaxy and is marked by sparkles.

Corporate Sector
The Corporate Sector is a free-enterprise fiefdom consisting of tens of thousands of star systems, it is run by a single wealthy and influential company, the Corporate Sector Authority (CSA). It was introduced in the Han Solo Adventures trilogy by Brian Daley

Placing New Planets
If an author's description of a new planet (sometimes described only in unpublished internal notes or outlines) does not reveal its general location on the map, the planet will usually not be placed right away. If and when it is decided to place the planet on the map (often because an author or editor has requested it, say, for a new version of a map), we will take into consideration as many continuity factors as possible:

Would it be close to a trade route?
How far from the Core would it be?
Is it alligned with any political faction?
What species are seen on the planet?

And yes, sometimes remote planets will be placed at a cartographer's discretion because there's a blank spot on the map where it sensibly fits.

And finally, I'd like to thank everyone who has participated in the discussions going on in the Galaxy Map thread. Your comments to this thread have made a tremendous impact in helping us make the galaxy maps better and better.