Get Berserker's and Vamp Scepter (or B.F. Sword if you can afford it) early. Get B.F. Sword and then build Zeal (should give more DPS due to Lulu's passive). Build BT for more sustain or IE for more scaling damage. Build Phantom Dancer or Static Shiv depending on whether or not you need wave clear. Build whichever BF Sword upgrade you didn't before.
Then you go into situational stuff.
The other Zeal upgrade for more ATS. GA/Mercurial/Warmog's for defense.
And then I usually don't get any farther than that when I ADC.
Generation 30: The first time you see this, copy it into your own signature (on any forum) and add one to the generation number. Social experiment.
What Metleon said but top. It's very similar to Kayle top in that you you are a massive lane bully and can poke like crazy. I actually prefer Lulu ADC top; as she has a much better lane phase and mid game as she brings so much due to the fact her support utility needs no items.
You have more CC and a shield so it's hard for gap closers to trade well. Many people struggle to do much after her poor burst as an AP; mostly cause her ratios are poor.