Lulu's got your back, and enemies should watch out for theirs....

Casting Whimsy on enemies during team fights:
Enemy characters that have strong attacks requiring a casting time can be interrupted by the Whimsy skill and are no longer such a threat. They are now adorable and fluffy. Great for team battles when your ally is running into their cast zone. Cast right when the enemy attempts to cast their skill.
Example of good targets for whimsy: Fiddlesticks ult, Zak's slingshot attack, Katarina's ult, Kaitlynn's ult, anyone who causes massive damage and / or has a cast time.

Casting Whimsy on Team:
When an ally is ahead of you chasing an enemy, casting Whimsy on your ally for a speed boost (and AP boost) can give them the extra oomph they need to catch up and make the kill. Casting "Help Pix" simultaneously will shield your ally as well as enabling you greater range to cast Glitterlance and slow the enemy even though you're farther behind.
When running away from an enemy, a cast of Whimsy and "Help Pix" on yourself or your team mate (Whoever is further behind) can keep you alive.

Solo Lulu Best Skill Combo:
Lulu has a powerful run and hit combo of skills that have landed me kills usually out of range for other characters. If the enemy has low hp (under 200) and is running just beyond range (especially beyond a tower) cast whimsy on Lulu for speed, then cast "Help, Pix" on the running enemy to extend your casting range and do some damage, then finish them off with a well placed shot of Glitter Lance from pix.
This also works if the enemy is hiding behind towers and there are creeps nearby. To avoid being tower hit, cast "Help Pix" on a full health creep (either ally or enemy) run out of tower range, and then cast Glitter Lance at the enemy.

When to use Wild Growth:
Team fights where an ally is surrounded by enemies is the most effecting time to cast Wild Growth, for extra hp, an knock into the air for enemies, and the slow movement effect on the enemy. Casting Wild Growth in lane when you or your ally need health and it would be risky to leave the lane.

Always be ready to help your lane ally. Do not use your 2nd and 3rd skills unless your ally is full or near full health, and you are at a team advantage and safe behind creeps.

Whenever there are team fights, stay near the edge of your range so your skills can stay helpful longer since she is low HP. She can do lots of damage even with her basic attack.

Never use your "Help, Pix" to attack enemies unless:
Your team mates are all at full health OR
The enemy is almost dead and you have all your skills available to make sure that you can catch them and succeed in killing them.

Harass:
Early game harass is very strong, a her basic attack is one of the strongest early game attacks. Using your basic followed with a cast of "Help, Pix" followed by Glitterlance once they are out of your usual range will take them by surprise if they are not familiar with Lulu's attacks. Don't forget you can attack them with this combo even under towers if you cast "Help Pix" on a full health creep that is within range of the enemy target.