Alright, 5e is coming and I have a concept (http://githyankidiaspora.wordpress.com/2014/08/15/daydreaming-about-a-hexcrawl/):

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Your maps are a thousand years out of date as you make your way to the surface...

It is one thousand years since the Drow exodus to the Underdark, soon after which the Web Matriarchs went to war against the Star King in the West. The Drow cut themselves from the fate dictated by the stars' song, bound themselves instead to the Matron's Holy Web.

Every nation was drawn in to this elven civil war. Gnomes, dwarves, drow, quaggoth and githyanki allied against elves, humans, hobbits and githzarai. On both sides, there were those who went against the majority. Undead were split – Ghouls and zombies made alliances on the surface while vampires and liches made alliances with the under-kingdoms. Orc were found on both sides.

Terrible arcane powers were unleashed that have left scars above and below, unleashing fell powers into the world. The gods sent visions to make it clear, this war would end the world if it continued. The holy gathered under a cessation of violence and crafted the treaty, binding a devil, angel and demi-god to enforce it. The treaty demanded that everyone choose above or below and stand by that choice for a thousand years: tunnels, stone and secrets buried in the earth or sun, moon and the heroes depicted in the stars.

Your ancestors chose tunnel, stone and secrets. In that thousand years, drow matriarchs have established vast city-states ruled by a complicated alliance of noble houses and priestesses. The githyanki attempted to take over the tunnels some years ago but were shut down by the combined might of their former allies; they are still present, though diminished – all of their red dragons slumber. The dwarves were driven into debt by the war effort and blamed their allies, always threatening to break the treatise as their princes and holdfasts faded from their former glory. The gnomes have quietly migrated deeper and deeper. The quaggoth ride underdark-bred dinosaurs, riding their steeds along ancient tunnel highways, following their mushroom herds.

Your maps are a thousand years out of date as you make your way to the surface...
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Local gaming buddies have been notified.

I've got a few pages of numbered hex maps printed out.

I'm glancing over these links:
http://arsphantasia.wordpress.com/2014/02/20/hexcrawl-resources/

I've got an hour and a half commute (each way), so here are the things I'm considering doing:

- Sketching out the map the players will have that is from 1000 years ago

- Making a timeline of major events from the past thousand years for Above and Below, especially things that would have an effect on the map and the surrounding country-side

- Considering the past 1000 year timeline of the Above, scar up the dated map, get my own copy up to date

- Look over Dyson Logos maps (http://rpgcharacters.wordpress.com/maps/), probably use a few of those as my first dungeons.

 - Make up randon encounter tables for different areas.

I'm not usually a hex-crawl, dungeon-crawling DM, so I thought I'd come here and ask for advice. Is there anything I am missing or should be considering as part of my DM's prep?

Thanks!

Don't get too wrapped up in the time line work. Just go over the major events for most of it and only really give any thought to detailing maybe the last 50 years. Also leave some mysteries to your self, for instance with certain historical events you may decide that there are only rumors of what happened, leave it that way even to yourself it gives you some flexibility.

When I am DMing sometimes I like to think of my notes as an unreliable narrator. I leave a lot of things open for interpretation at the table, it makes it easier to adjust for the twists and turns of player actions.

For a hexcrawl random encounter tables are a must. Make up some other tables too for quick on the spot generation, say for instance a random npc table or a random ruin generator.

Pick a starting area and go into detail on it. If there is plenty of interesting things for the players to get involved with in the starting locale they will likely spend at least a few sessions there. This will give you some time to flesh things out. When I am GMing I typically am only prepared ahead for the next few sessions.

Yup, the timeline will have space and not be too filled up.

Random ruins, random NPC's. Nice. Thanks.