Some options that jump to mind:
Libris Mortis has Corpsecrafter feats that bolster undead you animate. My favorite is Destructive Retribution; when you kill the animated undead, it explodes in a 5' radius negative energy burst. Which heals undead around it.
Eberron has some nice options for this:
1) The Bone Knight, who is a warrior PrC that can command undead. Give him some of your zombies.
2) The Dread Marshals are undead commanders; they can rebuke as a cleric equal to their HD, and can buff undead (make them do negative energy damage to those who attack undead within 60 feet of them, make the undead do damage, give them higher turn resistance, etc).
Don't be afraid to just mix up the undead a little. Instead of DR/Slashing, give those zombies Fast Healing 2. Or give them Regeneration (Fire and Acid).
But remember, zombies are cheap, easy cannon fodder. So they may not
intend to last long; their whole purpose might be just to be killed. Stick a swarm inside of them, so upon death they release a cloud of carrion flies. Fill them with poisonous gas. Cast Fire Trap on their mouths and instruct them to "Walk up to enemy and open mouth" (I had a necromancer make skeleton dogs and use them as kamikazi bombs like this). Maybe they're filled with diseases, so when they die they spread it.
If you're
really mean, slap this ability on your zombies:
Adhesive (Ex)
A mimic exudes a thick slime that acts as a powerful adhesive, holding fast any creatures or items that touch it. An adhesive-covered mimic automatically grapples any creature it hits with its slam attack. Opponents so grappled cannot get free while the mimic is alive without removing the adhesive first.
A weapon that strikes an adhesive-coated mimic is stuck fast unless the wielder succeeds on a DC 16 Reflex save. A successful DC 16 Strength check is needed to pry it off.
Strong alcohol dissolves the adhesive, but the mimic still can grapple normally. A mimic can dissolve its adhesive at will, and the substance breaks down 5 rounds after the creature dies.
Imagine the look on your Cleave-loving fighter's face when his sword gets stuck inside the zombie, while the others just crowd around him.
I agree with the above poster that just making them grapple the party is scary. Or (if they're intelligent enough?) use Aid Another. Also, as pointed out above, having Uncontrolled zombies just locked in a room works. They're 'reinforcements' that the spellcaster can Rebuke if their first wave gets killed.
Finally, nothing like mixing in some other undead into the group. Toss a single ghoul who goes after someone to paralyze them, so the other zombies can pile on top.
Oh, and slap a silence on one in the front ranks. Silence scares the crap out of casters. And utterly quiet zombies shuffling in unison?