During early laning, it is best for Varus to spec into either one of Piercing Arrow or Blighted Quiver for his preferred harassment. Blighted Quiver makes last-hitting much easier and lets him outdamage most carries early on during autoattack exchanges, whereas Piercing Arrow lets him snipe minions that would be hard to reach and is a superior harass from range. You should spec into Blighted Quiver if you can reliably get away with going into autoattack range and Piercing Arrow when you will be likely be strongly zoned.
Using Flash while channeling Piercing Arrow will not interrupt the spell. You can use this to finish off escaping enemies.
During skirmishes, it's sometimes best to fire Piercing Arrow quickly, rather than charging it to full power - although the ability gains range and damage with charge time, he cannot autoattack as he does so and Varus will likely be sacrificing a greater DPS if he takes too long.
Piercing Arrow has a distinct sound effect and causes Varus to change stance and slow down while charging it, making it easy to see coming. Try casting the ability from brush to keep the element of surprise.
Piercing Arrow gains more power and range the longer it is charged. However, the ability does not need to be channeled for the full duration in order to reach its maximum potential.
The range indicator for the ability will visibly expand once the ability begins to be channeled. Once the indicator stops growing, you will no longer gain any benefit for additional channel time.
Try to take advantage of Piercing Arrow's very long range to snipe at enemy champions before a fight or as they're trying to flee.
Varus can also very reliably steal buff camps with the ability. Most importantly, he can snipe the 20pxDragon from behind its cove wall with it even if it has been pulled out to the river by the opposing team.
When aiming for Varus' greatest possible DPS, you should employ strict cadence and fire off an ability after each three autoattacks on an opponent in order to detonate the most Blight stacks.
Remember that Blighted Quiver stacks on the enemy will only detonate when Varus connects with one of his other abilities, and all of his abilities have moderately long cooldowns - if you use Piercing Arrow and Hail of Arrows to initiate a fight, you will be left without an ability to proc Blight stacks for a short while.
Blight stacks will last for several seconds before expiring - if you have the chance, you can increase the efficiency of your large-scale attacking even more by hitting multiple targets sequentially and getting stacks on as many opponents as possible before detonating them all at once.
Since both skills scale with AD and are multi-target, Varus can effectively farm large waves of minions instantly late game using only Piercing Arrow and Hail of Arrows. He can be an effective pusher lategame if needs be.
Hail of Arrows is useful for helping you escape from or catch up to opponents due to the slow. It should also be noted that the healing debuff is very strong against champions that have heals such as Soraka.
Chain of Corruption has many uses - it can assist greatly in a gank by preventing the opposing team from fleeing, can cause massive disruption in a teamfight if it is allowed to spread, and can help ensure your own escape when on the run.
Remember than the disable from the ability is a snare, not a stun - ranged attackers and mages can still attack and use most abilities while they are caught. It can be much more effective to target any melee attackers instead, as they will be far more badly affected by the inability to move.
Trinity Force can be an unusual build choice for Varus - while it's high price will interfere with other carry item purchases, he benefits well from all of its stats and passive effects. In particular, Sheen procs can be consumed immediately after a Blighted Quiver detonation, giving Varus impressive burst potential.
Despite Varus' primary role as an AD carry, an AP/AS hybrid can occasionally be an effective, albeit niche, build. This style of play involves usage of the AP ratios on his Blighted Quiver to deliver respectable autoattack damage and has formidable burst potential from Blight detonations and Chain of Corruption.
A Lich Bane can add considerable damage to his burst immediately following a detonation.
A Malady can be an inexpensive boost to Varus' DPS in the midgame, as you will usually autoattack an opponent three times before proccing the Blight detonation with an ability. It's usefulness drops during the lategame, however, as it does not offer very many stats for an item slot.
Avoid being caught by Chain of Corruption. Spread out in teamfights to prevent it from spreading.
When Varus begins to charge Piercing Arrow, he will slow down and draw his bow back. He cannot perform any other action while in this stance, and his movement speed is reduced so take advantage of this time to juke his shot or close the distance on him.
Being hit by Hail of Arrows reduces your champion's healing and regeneration, so use healing abilities after the debuff wears off.
Varus lacks any escape while his Chain of Corruption is down. This makes him very vulnerable to jungler ganks.
Varus's Hail of Arrows only deals damage once on impact. When fighting Varus 1v1, don't be afraid of standing inside the Desecrated Ground unless the reduced healing/movement speed will significantly hurt your capabilities.