I love Necropolis, but I must be doing it wrong. The skeletons are incredibly weak at the start, and by the time zombies hit anything, it's had time to go off, start a family, and send its grandkids back to fight. The wraiths are okay but not outstanding. That's what I think so far anyway. I have played a bunch of times, but always found the start very hard on Necropolis.
How do you guys start somewhat strong?
Do you have any rules of thumb for when to upgrade regular skeletons?

Yep, what blueskirt said. Expert Earth and Wisdom provides Town Portal by choice and mass slow. it also makes animate dead permanent (if it wasn't already--I forget). Plus, implosion is an Earth spell, so it makes that very powerful as well.

In my opinion, regardless of the individual hero, the absolute best skills are Earth Magic, Air Magic, Wisdom, Logistics, Intelligence, Navigation, and Eagle Eye. Okay, I'm kidding about those last two, but Earth/Air/Wisdom/Logistics/IQ give the hero the ability to cover the whole map each turn, and provide very powerful spells for combat along with the mana to handle it all. Once you start capturing towns and getting their spells, your hero becomes very powerful and fun to play.

Don't forget the importance of Pathfinding. Even a hero with expert Logistics will move at a snails pace through swamp/desert/snow without Pathfinding. Water Magic is also a very useful school, possibly even more useful than Air Magic. Mass Forgetfulness pretty much completely negates enemy archers, the mass buffs/debuffs (Prayer, Bless, Weakness) can really help out in tough battlers, and mass Cure is invaluable if an enemy hero is casting a bunch of debuffs on your troops.

You make good points. My preference is to give Earth & Log. priority, and then let the others come as they may, and with Air, I can bypass bad terrain with Dimension Door; I always feel like the Hulk jumping around when I open door after door. Mass Cure/Forgetfulness are quite effective, but I like knocking suckers out before it becomes an issue.

Again, it comes down to personal preference. It's best to try them out to find what works best for you--such a great game with so many gaming possibilities!

Depends on the map you are playing but the following works for me in general and on random maps:

Week one: Town Hall, Mage guild, Marketplace, Blacksmith, City Hall, Citadel, Castle

Week two: Capitol, graveyard/zombies, wights, Mansion/vampires, Mausoleum/ liches, skele converter, Open graves(+ 8 skeles/week).

Week three: Black Knights, Bone dragons, Necro-shrine(increases necromancy skill & needed for vampire upgrade), upgraded mansion, mage guild 2, upgraded skeletons, upgraded zombies

Points to remember:

1)Get to the Capitol in your main/starting town ASAP! You are not likely to be attacked in the first few weeks(assuming large/X-large maps).

2)Your first unit upgrade should be the vampires to vampire lords. After that it is up to you and does not matter terribly much except I like to get the Skeletons and such up to speed before increasing the firepower of others like liches. Avoid lugging around slow units as much as you can.

3)Do not take zombies with you unless you absolutely have to. They are too slow and work far better as town-garrisoned defenders. And you would rather have two stakcs of skeles than one stack of zombies + one skele. This is not only faster but also leaves you able to pick up recruited(through Diplomacy or luck) unit stacks by combining the two skele stacks to make room.

4)Attack evenly matched or weaker stacks on map quickly and often. This gives XP and increases army size(skeles anyway...at least until you get the cloak of undead king). Always choose to fight those who want to flee. But be wary of ranged stacks in the early game as they can deal heavy losses to your troops before they make it across the map to deal with them.