I play her in every role but ADC. ^^
Anyway, top is about knowing your opponent's kit, and knowing how to abuse it with yours. You have to be able to bait people into wall stuns, then you beat their face in and get out before they come out of the stun. When you get your ult, you stun then, throw Q, and then tap R as soon as they come out of the stun, and it's just a ton of damage and CC -- then follow up with another Q.
Now, unlike most people, I don't build her tanky. I build her like a bit of a bruiser-assassin mix, and just melt people. Start with a Sapphire Crystal, rush a Sheen and armor or MR boots, then build BC > Triforce > Dead Man's > Maw (if they have AP) / Sterak's (if all AD) > BORK. If you need to be tankier, swap the BORK out for Sterak's.
As for combos, E>Q is the standard one, and you can even do it without a wall stun if you can kill them. My favorite combo is W>Passive+E (animation canceling voodoo here, super cool)>Q>R when the stun is done>Q as soon as you can. This combo will kill anything but the tankiest champs, and for them, you just need to chase them a bit and try for another stun. :P
Poppy is a lane bully. So if you can survive ramming them into a wall repeatedly, do it! Push them out of lane. You can also drop your Q on the minion wave and tag them with the very end of it, while farming. You don't absolutely have to pick up your shield either -- it's really only good if you want to get in their face and stay there. Otherwise, just ignore it.
Your shield also applies on-hits, like Dead Man's, Triforce, Frozen Mallet, red buff, and all that. So use that to your advantage!
As a final note, don't be afraid to tower dive if you know you can kill them (you can stun people on towers). The passive effect on your W makes it easy to survive the shots, and the MS boost from activating it gets you out before you die.