Secondary professions for a Warrior

Warrior/Ranger

 * The Ranger line offers a few alternative IAS skills.

Warrior/Monk

 * Skills such as Mending, which has a small return for a high cost of attribute points and Orison of Healing, which is a mediocre heal, should be avoided. Although when used with skills like Warriors Endurance the energy degeneration can be avoided completely.
 * Mending Touch is a useful form of condition removal, even with little or no attribute point investment.
 * Supporting skills such as Vigorous Spirit provide excellent healing over time, allowing the Warrior to keep on fighting. In addition, Vigorous Spirit can combine well with Cyclone Axe and Triple Chop.

Warrior/Necromancer

 * The most common usage of the Necromancer secondary is for condition removal via Plague Touch.

Warrior/Mesmer

 * Wise use of Physical Resistance, Elemental Resistance or elemental mantras can add additional survivability to solo or tanking builds.
 * The skill Echo, in conjunction with certain warrior skills (such as "Fear Me!" and Sun and Moon Slash, can produce great effects.

Warrior/Elementalist

 * Shock can be used as a snare and interrupt, however exhaustion will certainly be felt with a Warrior's low energy pool.
 * Grasping Earth can be used as an AoE snare to keep groups of enemies from kiting as efficiently.
 * The conjure skills, such as Conjure Frost can provide additional damage when wielding an elemental weapon for a very small investment of energy.
 * Defensive Earth Magic enchantments such as Obsidian Flesh and Stoneflesh Aura can be used for tanking in high-end areas such as the Domain of Anguish, but maintaining them may require some energy management beyond your normal 2 pips of regen.

Warrior/Assassin

 * The Assassin has a wide range of snares and shadow steps that can serve a Warrior well. Shadow Prison for example.
 * Signet of Malice can provide free condition removal.
 * Dash is an excellent speed boost that also serves as a quickly recharging cancel stance for Frenzy.
 * Death's Charge is useful for closing distance in battle, but take care not to outrun your party's support.
 * Disrupting Dagger is a useful interrupt which cannot miss or be blocked.

Warrior/Ritualist

 * Weapon spells can benefit a Warrior; Splinter Weapon can be combined with Triple Chop or Hundred Blades, Brutal Weapon adds overall DPS (especially when combined with the excellent warrior IAS skills), and others can add interrupts or other benefits to attacks.

Warrior/Paragon
Warrior and Paragon skills have very little synergy.

Warrior/Dervish

 * A scythe can work in a Steady Stance/Drunken Blow/Desperation Blow build.
 * Many Scythe Mastery skills are general-purpose Melee Attacks (i.e. Rending Sweep) rather than weapon-specific skills, which allows them to be used with axes, hammers, and swords.
 * Rending Touch can be used as an enchant removal.
 * Pious Haste can be used as a cancel-stance for Frenzy.