First of all, Lulu is a great support. But, she can be a great AP Carry aswell. I've played her a bit and I started to think which items would benefit her as a Mage. For starters, Sheen would give her more damage to her autoattacks, and second, Rylai's Crystal Scepter would give her more slow and more HP to survive those teamfight where you are focused.

I know this guide isn't so informative, but it's my first one, and I really wanted to share with you my thoughts of Lulu. I hope it will be helpful to you and I'm open for suggetions.