When I smoke and my parents are home, i usually just do it in my room. Get a semi-wet towel, and wedge it into the crack of your door. Light some incense or a candle for a good 10 or 15 minutes before you start. Then, open a window, and if you're still worried, get a fan. Blow smoke out of the window. Get a real pipe. They're not that expensive. Make sure you take a shower before you do this, and if a parent tries to come in while the towel is still under the door, the towel also doubles as a method of slowing their entrance. Wedge it in there real tight, and you'll have to let them in, which gives you time to hide anything left out in the open.

I come home high all the time, it just takes practice. Never underestimate the power of denial, if it's on your side. Invest in some eye drops, and some strong gum. Dress in layers, and remove them as needed if you reek of the ganja. Never exhale into the wind.

As far as incense goes, I started burning it out of complete innocence when i was young. My parents have come to see incense as a normal occurence in the house. Now that I look back at it, it must have been my stoner instinct. But anyway, start burning incense in the bathroom as needed. After a few weeks of this, move it into your room and burn it when they're around and you're not doing anything. If you can make it into a routine, you'll always be able to claim innocence.

I think that's it... good luck.