I'd look at other mega dungeons and see what you liked or didn't like.  For example, I consider the Dragonlance series a mega dungeon.  While the idea was intriguing and the amount of effort that went into it amazing, the end effect ended up being less than the sum of its parts for some reason.  On the other hand, the GDQ series seemed to be greater than the sum of its parts despite some major short-comings (like is Lolth really the "bad guy" or was it Eclavdra?)  What makes a mega dungeon mega good or just mega big?  I hope that helps.  

Some good discussions about mega-dungeon/megadungeon/campaign dungeon creation at:

Dragonsfoot @ http://www.dragonsfoot.org/forums/viewt ... 15&t=18710

Knights & Knaves @ http://www.knights-n-knaves.com/phpbb/v ... m.php?f=28 in general and specifically @ http://www.knights-n-knaves.com/phpbb/v ... .php?t=168

Fin's OD&D in general @ http://odd74.proboards.com/index.cgi?board=adventures

Philotomy's OD&D musings @ http://philotomy.com/ include several good pieces on dungeon design

And the following blogs are worth checking out too (in no particular order):

http://poleandrope.blogspot.com/
http://oldguyrpg.blogspot.com/
http://grognardia.blogspot.com/
http://beyondtheblackgate.blogspot.com/
http://greyhawkgrognard.blogspot.com/
http://kellri.blogspot.com/

There are plenty more good old school blogs out there, but if you're designing a mega-dungeon, I think the above will be most useful as design resources.

I think bejoshua brings up a good point.  Depending on the type of megadungeon you plan on creating, you have to look for different answers.  One of the things that makes the GDQ series great was the aspect of an actual journey.  Unlike something like Undermountain which is all in the same geographic location, GDQ promoted planning.  The Dragonlance campaign, while well crafted, felt like you were on rails without any choice in how things would end.

If you needed a break from a static megadungeon, you just waltz back to town and get healed/resupplied.  If you're out in the middle of nowhwere, you are forced to plan for that.  It adds something to the adventure, both from a player and DM aspect.  

Dynamic areas add to the whole experience and should be used if possible.  While making everything "special" makes everything the same, some "special" should be used to make the adventures memorable.  

Just some thoughts.

I start with the maps.

My review of Castle Zagyg may also provide some useful food for thought about was good/bad about that product, and the other products that I use to contexualize my thoughts on it: http://www.greyhawkonline.com/grodog/gh ... _final.pdf

Agent Cooper wrote:http://www.worldslargestdungeon.com/archive/

This really is the biggest piece of crap anyone can buy. I've never heard anyone try to defend it, and I doubt even the authors' would be willing to do so. Stay well away from The World's Laegest Dungeon.

Gus, have a look at a series of articles called 'the Dungeon Architect' in early issues of White Dwarf.  Drop me a line if you don't have access to it.

Wow, thank you all for the advice! I've got my work cut out for me with all this prep work.

grodog - thank you for all those links. Lots of great stuff!
otherworld - thank you for the suggestion. I'm reading the articles now!

At anything more than $20, you may end up feeling like you were royally screwed over.And that includes shipping.