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.
This makes laning with Soraka more viable, as she can replenish mana.
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.
However, it is still useful for picking off enemies after a team fight.
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 there are less then 5 minions and 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 heavily mess up combo-reliant mages.
Spell Shield has a long cooldown when it's still rank 1, so use it wisely during early game. Because when using Spell Shield to regain mana, you are vulnerable to crowd control effect.
Sivir is fairly squishy in the lane. Putting a pressure on her can make her underfarmed or underleveled.
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 the spell's damage.
Also, keep in mind that Boomerang Blade will return to Sivir after reaching its max range. Being at it's 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 希维尔的Spell Shield 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. This will cause her to use up a huge chunk of mana, and put the shield on cooldown. Doing this 2 or 3 times can leave her manaless and unable to cast her spells, which is a great advantage to you as she's very ability reliant.
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.