Don't MOOP - and BE A MOOPER!

At this point in the game, you should have read your Survival Guide and (if you're a newbie) the First Timer's Guide.  If you haven't, go do that RIGHT NOW!
Survival Guide: http://survival.burningman.com/
First Timer's Guide: http://www.burningman.com/first_timers/

Having read these two very important guides, you are now savvy about Matter Out Of Place, or MOOP.  MOOP is defined by ANYTHING that is not naturally part of the playa.  If it's not dust or a rock, it's MOOP.
The Leave No Trace tenants of Black Rock City apply on every level; our goal as Burners is to keep the playa clean and leave it better than we found it.  Your goal personally is to "never let it touch the ground" - and if it does, pick it up immediately.
These concepts are all to the good, but I feel that most MOOP occurs accidentally and without our notice.
Obviously I didn't mean for the basket on the back of my bike to spring a hole and leave a trail of fun items in the dark & dust behind me while I rode around (my first year - 2007 - and I lost a lot of things I'd rather have kept!)  That giant roll of gaffer's tape I found half-buried in a dust-dune in '08 was obviously a bummer for whoever dropped it (score for me, though!)  My point is that MOOP happens.  So what can we do about it?
First and foremost, take care of your camp and possessions.  Obviously my bike cargo basket in '07 was not sufficient.  Since then I take extra pains to secure anything I carry in or on my bike.  I added a large drawstring bag to my bike basket, using it as a liner to keep small items in - and it doubles as a dust-shield for items within.  Since that improvement I haven't lost anything out of my basket - at least not that I'm aware of.
The same idea goes for your camp.  It's easy to spread things around - my camp is usually full of things like art supplies, costume pieces, gifts, and misc. playa-fun tidbits.  It's smart to keep everything stored in bins, and when you take something out be sure to put it back as soon as you're done with it.  This will save time later, when you go to leave camp and realize things are a hot mess.  Every time you leave camp, make sure everything is put away and secured.  Zip up your tent, lock your car, and pack up anything and everything that could blow away.  If you have open trash bins, cover them and secure the lids.  The wind & dust storms kick up quickly and sometimes with no warning.  It might be beautiful and calm when you leave camp, but an hour later when you're on the opposite side of BRC the conditions could change and leave your camp a wasteland.  MOOP that gets blown out of your camp can quickly get buried in loose playa dust, litter other camps, and in the worst-case scenario become a danger to other Burners as it flies through the air.  This goes for food and eating supplies as well - I've seen paper plates rolling like tumbleweeds down the roads, moving too fast for me to catch.  Once something is riding the wind, it can be really difficult to catch it!
An excellent example of this is the video posted by noonatron008:
http://www.youtube.com/watch?v=hzT2qyMZJ98
- and no, he never did find the hat!

Which brings me to my next MOOP-related topic: costumes & MOOP.
You are just as responsible for your fabulous clothing as you are for your camp and other supplies.  A good pre-Burn costume check is to take a questionable item and shake it vigorously for 30 seconds.  If nothing falls off of it, you're probably ok.
Feathers are NOT ALLOWED on the playa in any form - headdresses, earrings, etc. included.  Feather boas are especially MOOPy.  This is a hard one for me - I'm a costumer and I do love embellishing with feathers - but we try to be good Burners and I have cut them out of my playa-prep pile.  Gate security can deny you entrance to BRC if you have feathers in your vehicle.  Be warned!  Glitter is also another playa-no-no; loosely secured sequins as well.  Though they may look great on your skin or your vest, they will eventually fall off you and into the playa, where it's nearly impossible to remove them.  Leave these items at home!
Aside from the obviously MOOPy items, be aware of other aspects of your clothing that might come loose or fall off.  The guy in the video chasing his hat obviously had no intention of losing it, and like a good Burner did his level best to retrieve it.  Wide-brim hats are a necessity during the day, but they like to blow away too.  Try tying a scarf around your head and hat, or secure it with a string or ribbon under your chin.
Jewelry is another big unintentional MOOP item.  Earrings fall out, necklaces fall off, faux gems fall out of rings...  Try to wear accessories with strong hardware.  I'm a fan of those little rubber backs that fit on french-hook earrings to keep them from sliding out.
Glowsticks are popular for nighttime safety bling, but one the of #1 MOOP items collected by the clean-up playa restoration teams post-Burn are the little plastic connectors.   Not all glowsticks are made the same; for some reason, some of the types on the market don't work that great and pop out of the connectors.  The glowstick is usually easy to retrieve, but those little bits of plastic go flying.  If you must use glowsticks, make sure they aren't going to go spouting MOOP once you start moving around.  Either get the good ones or try reinforcing them with a bit of tape.
Always be thinking of ways to secure things to your person.  Carabiner clips are endlessly useful, as are pocket belts with grommets or D-rings that you can affix things to.  Be prepared and be aware, and you too can reduce MOOP on the playa!
Below is the official "MOOP Map" posted by the playa restoration crew after the Burn in 2009!  '09 was one of the least-moopy years in Burning Man history.  The red and yellow spots are the worst for MOOP - the areas where the big dance camps are at 10 and 2 o'clock especially.  Let's try to have an all-green MOOP map for 2012!

And finally: BE A MOOPER!  A "MOOPER" is a Burner who always picks up MOOP, wherever he or she finds it.  Everyone should carry a MOOP Bag - I usually just use a big ziplock, but anything will work as long as it's secure. Anytime you see MOOP on the playa, pick it up and dispose of it properly.  If you're up all night dancing at that amazing big sound camp, help clean up in the morning when the sun comes up.  The big dance camps are the worst for MOOP because of all the party people that unintentionally shed MOOP while dancing, or are just too spun to clean up after themselves and leave behind bottles and other items.  You do your community a great service by picking up items to return to lost & found, or recycle, or dispose of.  Plus: it's good karma - something that's always good to have on your side, especially on the playa!  Some friends of mine even create interesting playa-gift jewelry from pieces of reusable MOOP every year.   And you might get a lucky ground-score in the process!  It's a win-win!