Boosts

Stacking Boosts

Boosts of the same kind don't stack.
For example, if you equipped the Cape of Awe (25% XP, CP, Gold, Rep) and then the Dark Summoner armour (15% Rep), you would have 25% XP, CP, Gold and Rep overall boost, as the game follows the highest boost equipped.
However, consumable boosts, server boosts, and gear boosts will all stack with each other, even if they're the same kind. For example, you could mix a 48hr XP server boost, a 1hr XP consumable boost, and the Cape of Awe. This would be the best time to farm if you can align all three.
What about damage boosts?
Similar to above, you wouldn't be able to stack two damage boosts that do the same thing. However, you can stack items that boost damage to All with something else that boosts damage to a tagged enemy. For example, if you equipped the Void Warlock Armour (25% All) with the Blinding Light of Destiny (75% Undead), you would deal 118.75% extra damage against Undead enemies and 25% damage against every other enemy.
How do boosts stack?
You may be wondering why the previous example of stacking the Void Warlock Armour and Blinding Light of Destiny (BLoD) resulted in a 118.75% boost. Why not 100%? This is because stacked boosts are calculated multiplicatively, not additively.
For the Void Warlock Armour and BLoD, it would be calculated like this: 1.25 x 1.75 = 2.1875

Anything else?
Damage boosts do not affect DoTs (Damage over Time) or HoTs (Heal over Time).

Best Mixes

Acronyms

Best Overall

EA or NSoD + RGoW =
Non-Tagged: 51% Boost
Chaos, Elemental, Dragon, Undead, Human: 126.5% Boost
Gold: 30% Boost
CP, Rep: 50% Boost

Best Individually

Use these mixes for the best single monster-type damage boosts:
Chaos: AFDL or IFP + Empowered Chaos Avenger's Greatsword = 136.25% Boost
Undead: AFDL or IFP + Blinding Light of Destiny = 136.25% Boost
Human: AFDL or IFP + Darkon's Debris 2 (Recontructed) = 136.25% Boost
Dragons: AFDL or IFP + Legion DragonBlade of Nulgath (needs membership to obtain) = 136.25% Boost
Elemental: AFDL or IFP + Infernal Flame Khopesh = 136.25% Boost
(there are other options for 35% All armors or 75% Racial weapons, these were just used to demonstrate the numbers)
If you don't own AFDL or IFP, use these instead:
Chaos: EA or NSoD + Empowered Drakath Armour = 126.5% Boost
Undead: EA or NSoD + Empowered Undead Champion = 126.5% Boost
Human: EA or NSoD + Sepulchure's DoomKnight Armour = 128.01% Boost
Dragons: EA or NSoD + Fire Champion's Armour = 126.5% Boost
Elemental: EA or NSoD + Malgor the Shadowlord = 126.5% Boost
(there are other options for 51% All weapons or 50% Racial armors, these were just used to demonstrate the numbers)
If you don't own EA or NSoD, you could replace them with:
A 40% All weapon (Void Warlock Weapons, Awescended Omni Armblades, Hollowborn Reaper's Scythe)
A 35% All weapon (Iara Merge Items)
A 30% All weapon (Burning Blade of Abezeth,Thaumaturgus/Apostate Ultima,Hollowborn Oblivion Blade, etc.)
A 15% All weapon (Burning Blade, Nightlocke Weapons)

All (relevant) Damage Boost items

51% All Weapons

Multiple Racial Tags Items

75% Racial Weapons

50%/51% Racial Armors

Undead: Empowered Undead Champion
Dragon: Fire Champion's Armor
Human: Sepulchure's Doomknight Armor, Void Beetle General
Elemental: Malgor the Shadowlord
Chaos: Empowered Chaos Avenger Armor, Empowered Drakath Armor, Empowered Original Drakath Armor

35% All Armors

40% All Weapons

35% All Weapons

30% All Weapons

15% All Weapons

All (relevant) Non-Damage Boost items

30% Gold, 50% Class/Reputation Points

25% Gold/XP/Class/Reputation Points

20% XP