During the laning phase, you can harass enemy champions with Boomerang Blade to put pressure on them. Successful hits will force them to play safer and get zoned out. However, the mana cost of this playstyle is extremely high, due to Sivir's small mana pool.
Additionally, Boomerang Blade returns to her after reaching the max range, so you can move your own position to hit enemies who would otherwise have evaded it.
Boomerang Blade also reaches a little farther than what the range indicator shows.
Boomerang Blade can be used to check for enemy champions in the brush, due to its slicing sound when it hits an enemy unit/champion.
As Boomerang Blade deals less damage with each enemy hit, try to hit champions without hitting minions.
A good strategy is to wait until a wave clears, or when they are running in a straight line.
Boomerang Blade tends to fall off somewhat later in the game compared to Sivir's autoattacks.
However, it is still useful for picking off enemies after a team fight.
Despite being a champion with relatively high mana costs and effective lane zoning, Sivir can easily and effectively be adapted to play in an aggressive lane with supports such as Thresh and Leona with a few simple changes to mentality.
Use Spell Shield to bait out important spells from your enemies before you engage, OR use spell shield to bait an engage out of your opponent. Only do this if you and your support are prepared to fight to the death.
With this in mind, DO NOT mindlessly waste Spell Shield. Only block spells if you need mana or if you don't fear any subsequent spells from the enemy. Players are aware of Sivir's need to regenerate mana with her Spell Shield and may bait her into blocking a damage spell in order to hard engage her and win an easy kill.
Boomerang Blade is a zoning tool by nature, but can be easily wasted if you only use it for this purpose. Instead, conserve mana until you are ready to hard-engage the enemy, then burst down your target with a mid-range Boomerang Blade followed immediately by one Basic Attack/Ricochet reset combo. This entire combo should hit before the Boomerang Blade returns to deal its return damage.
This combo allows Sivir to compete with lane bullies and bursty carries such as Lucian in a hard engage, but the Boomerang Blade can still be used for extreme range control and zoning if necessary.
Ricochet is a cheap tool for harassment. Try to hit enemy champions with it directly after an auto attack, or last hit creeps with a Ricochet when the enemy is close to the wave.
Ricochet also resets your attack timer, which means you can use it immediately after you auto-attack to get a "free attack."
Getting at least one rank of Spell Shield early is very helpful, as with a bit of practice you will be able to use Spell Shield to negate spells. With experience you can avoid damage/crowd control/death and recover mana in the process.
Spell Shield is especially strong when laning against mage champions, particularly ones with delayed damage spells, as it allows you to regenerate mana fairly often. It can also disrupt combo-reliant mages.
Spell Shield has a long cooldown when it's still rank 1, so use it wisely during early game. When using Spell Shield to regain mana, you are vulnerable to crowd control effects.
Spell Shield enables Sivir to lane mid as it can counter most spells while also regenerating Sivir's mana.
The armor-shred passive of The Black Cleaver is triggered by not only auto-attacks, but on any effect which deals physical damage such as Ricochet and Boomerang Blade (for each pass through an enemy). This can apply max armor-shred stacks very rapidly across an enemy team.
Since Sivir can easily push using Boomerang Blade and Ricochet, Getting Phantom Dancer in place of a Statikk Shiv may be fairly better choice. It provides 10% more attack speed and Critical strike chance, but 1% less movement speed than Shiv does. The unique passive works well with Sivir's Fleet of Foot, since it allows you to chase without the hindrance of unit collision.
Some ranged AD carry champions tend to rush Infinity Edge, however The Bloodthirster will provide more reliable damage on Q-W combos, as well as sustain.
An early Manamune can solve mana problems, but will reduce early laning-phase DPS and is thus ultimately nearly useless due to your Spell Shield generating its own mana when it is used up.
Sivir is fairly squishy in the lane. Putting a pressure on her can make her under-farmed or under-leveled.
Keep in mind that she is very resilient to zone because of her Spell Shield, and being too aggressive might make it easier for her to land Boomerang Blade.
Sivir deals entirely physical damage, and much of that damage is dealt through autoattacks. Consider buying armor if she begins to deal too much damage.
Avoid taking free damage from her Boomerang Blade, by moving out of its path.
If you can't dodge it, staying behind your minion wave will mitigate some of the damage.
Also, keep in mind that Boomerang Blade will return to Sivir after reaching its max range. Being at its max range could be advantageous in that it's a slow projectile, making it easier to dodge, but could also be a disadvantage due to the projectile hitting both the throw and return strike very quickly.
Wait until Spell ShieldSivir is on cooldown before activating your high-damaging spell. Bait her into using it by acting as if you're going to cast on her, but don't cast.
Sivir is often very attack speed oriented. As a tank, grabbing a Frozen Heart or Randuin's Omen can be a very effective counter her, and champions like Malphite and Gragas, who can slow attack speed, are also nice counter-picks to her.
Although Sivir is a carry, she is still fairly powerful early game. Her Boomerang Blade hits hard if she lands both portions on you exclusively, and her Ricochet and Spell Shield are specialized for one-on-one skirmishing. Taking her head-on without heavy armor or hit points can result in a long trip back from your nexus.