Spider Form abilities do not cost mana and can be prioritized when you are trying to conserve mana.
Use Spider Form when escaping, as being in this form grants you 10 bonus movement speed, your spiderlings will trail behind and possibly block skillshots, and you can use this form's Venomous Bite and Rappel to gap close away to enemy minions or neutral monsters.
Venomous Bite will still cause you to dash to the target if it dies during the cast time, but the spell will not go on cooldown.
A neat trick with Volatile Spiderling during the laning phase is to aim the skill towards your turret or away from the enemy minions, followed up by a quick Venomous Bite to an enemy champion. The created spiderling will jump to your target, ensuring the skill will hit.
Use Spider Form to get back to lane faster, as it gives bonus movement speed.
Rappel can not see stealthed champions, however it can see those in brush, when suspended in the air.
Rappel can be used to get over terrain, as long as Elise can Rappel to a target.
On-hit build (Nashor's Tooth and/or Wit's End) also works very well as her spider form grants bonus magic damage, and her Skittering Frenzy is a very large attack speed boost. However doing this makes her human form less effective.
Lich Bane is an effective item for Elise, as it grants some damage after every spell.
Zhonya's Hourglass is another defensive item which gives you more damage. Also its active ability can save you in difficult situations, synergizing well with the short time of disappearance from your Rappel. The spiderlings aren't affected by the active, so they can inflict some extra damage during the stasis.
Due to how spammable her abilities can be late game, building Rylai's Crystal Scepter on her would allow you to constantly slow your enemies especially in human form and increase your chances of survivability.
Rappel will only move Elise straight up and down unless she can descend upon an enemy unit (including minions and neutral monsters).
Rappel has a long cooldown and she is vulnerable after she has used it.
Rappel can be cancelled if you use hard crowd control on her (such as for example the knock-up from Jarvan IV's Dragon Strike) while she's on her rising animation, and it will cause her to immediately descend upon rising.
When you are targeted by Elise'sRappel, she can be lured into turret range or other dangerous areas by moving there when she starts to go up, as she cannot cancel the ability once cast. A quick Flash or dash ability is the most effective way to do this.