Pile on some heavy Lightning and Fire resists along with something that grants the Cannot Be Frozen (alternatively, you can gulp down some Thawing Potions prior to fighting Diablo, as these grant the same CbF along with +50% Cold and +5% Maximum Cold Resists).

The following low-level items grant CbF:


Death's Guard
Sash

Defense: 22 (Base Defense: 2)
Required Level: 6
Durability: 12
8 boxes
Cannot be Frozen
+20 Defense

The Completed Hsarus' Set (Buckler, Belt, Chain Boots) offers:

Attacker Takes Damage of 5
Cannot be Frozen
Lightning Resist +25%
+5 to Max Damage

The Spirit Shroud
Ghost Armor

Defense: 295 (Base Defense: 102-117)
Required Level: 28
Required Strength: 38
Durability: 20
+150% Enhanced Defense
Cannot Be Frozen
+1 To All Skills
Replenish Life +10
Magic Damage Reduced By 7-11 (varies)

Duriel's Shell
Cuirass

Defense: (528-650) - (610-732) (varies)(Base Defense: 188-202)
Required Level: 41
Required Strength: 65
Durability: 150
+160-200% Enhanced Defense (varies)
+ (1.25 Per Character Level) 1-123 Defense (Based On Character Level)
+ (1 Per Character Level) 1-99 To Life (Based On Character Level)
Resist Fire +20%
Resist Lightning +20%
Resist Poison +20%
Resist Cold +50%
Cannot Be Frozen
+15 To Strength

Darksight Helm
Basinet

Defense: (77-273) - (86-282) (varies)(Base Defense: 75-84)
Required Level: 38
Required Strength: 82
Durability: 30
+ (2 Per Character Level) 2-198 Defense (Based On Character Level)
6% Chance To Cast Level 3 Dim Vision When Struck
Level 5 Cloak of Shadows (30 Charges)
Cannot Be Frozen
Fire Resist +20-40% (varies)
5% Mana Stolen Per Hit
-4 To Light Radius

Personally, I would recommend Duriel's Shell.


Diablo's Lightning Storm also causes physical damage, so if you have anything that grants Reduce Damage by x/x% than that will also help increase your length of life.