1: Buy big huge units, and keep summoning!
It's always nice to start with a nice, meaty unit of, say, 30, but you'll find that with skeletons (and especially zombies) that number can quickly drop to nothing. So, make sure you keep one of your necromancers close by and you should be raising new troops every turn. Well, unless you're going to cast "curse of ages" on someone... oh, and by the way, I'm always tempted to make my units 5 across, but you only need a row of 4 across to get a rank bonus- and since rank bonuses are your best friend...
2: Never rely on zombies...
Early on, way back before the 6th edition Vampire Counts army book came out and before zombies REALLY started to suck, I decided to protect my general by sticking him in a 20-figure unit of zombies. Because I had a lot of them. On each side of that unit, I had a pair of ten-skeleton units, ready to flank anyone who tried attacking them.
Unfortunately, the army I was playing against was chaos, who cut a great bloody swath THROUGH THE ENTIRE UNIT AND THE GENERAL. Needless to say, I wasn't pleased.
3: ...But don't be afraid to use them.
Probably the handiest use of zombies are as decoys and speed bumps. You can raise them in the path of extremely large units to buy yourself an extra turn to get some of your more powerful units where they're needed. Or -and this is my favorite tactics- raise them right in front of a unit of night goblins- they'll draw out the goblin fanatics while your main units are still out of range. Sure, your zombies won't survive the turn, but hopefully, neither will the night goblins once the fanatics start moving randomly.
4: Fear the Lizardmen
I have only played two games against the lizardmen, one which I lost, and another which I would have lost if I weren't allied with a chaos player. Their special leadership rolls can spell doom for your force, which relies heavily on scaring the heebie-jeebies outta your opponents. Beating them requires a completely different army setup, too- buy a lot of wights, take all your special slots and most of your points if need be. And make sure you have large core units of skeletons handy- buy the maximum size if you can afford it- you'll be relying on rank bonus and outnumbering far more than usual. If anyone has a VC army list that does well against the lizardmen, I'd really like to see it.
5: Protect your general...
Well, duh. With an undead army, this is particularly important, though- once the general snuffs it for the final time, your entire army will collapse slowly. And keep them away from the front, too, unless you're extremely confident that either it's REALLY good at fighting, or your opponent's incompetent. I was allied with chaos at one point against the wood elves and the bretonnian armies, and my general -a lahmian- was protected by an honor guard of three dragon ogres.
The dragon ogres were annihilated by a charging bretonnian force- that trampled right over my general. Oops.
6: ...but don't be stupid about it.
A protected general is a living general, however, if your general is safely hidden waaaay at the back of the battlefield, your units will suffer, moving at half speed, and out of range of most, if not all, of your spells. Be paranoid with your general, but don't be stupid about it.
7: Be prepared.
If you're going to be playing against some cheap elf army with millions and millions of bows and mages, don't fill your army with slow-moving troops- come prepared with a few alternatives. For example, what would you replace your skeleton troops with? Bat Swarms? Ghouls? Both count as skirmishers, making them harder to hit, and the bat swarms can move 10". Fell Bats are skirmishers as well, and can move 20". Dire Wolves aren't skirmishers, but can still march or charge an impressive 18" and cause a lot of damage when they're close. And, if you have room, why not a unit of Wight Knights and a Necromancer on horseback to help him keep pace with the army? And remember, if you can, raise straight lines of zombies to block the line of fire of any riflemen or archers. They'll probably die before the end of the turn anyway, but that just means they won't be in YOUR way.
8: Ethereal Stuff!
I'm not kidding when I say ethereal stuff rules. Immune to artillery and firing lines, but they can still be used to block the line of sight of your more corporeal units. The banshee, though, is without a doubt the best. Ethereal, terror causing, and can bypass armor with her scream. Take that, chaos!
Tactics of Vampire Counts: