When you rent a car via the online car rental reservation process, you are never told about THE MANDATORY UNPLEASANT BONING (all caps are necessary, yo). I am here to warn you about it, because if you are prepared for THE MANDATORY UNPLEASANT BONING, at least you can lube up first.
Here's what happens:
1) You reserve a car online from some car rental place at the airport. Normal. All good.
2) You show up at the airport, and you go to the rental counter, and they run your credit card and all that. Normal. All good.
3) You walk to your car, and you load your crap in the trunk, and you then get in the car and fasten your seat belt. Normal. All good.
4) and THEN (dun dun DUNNNNN!) you notice the passive-aggressive note that they left for you in the car that says, "Oh yeah, um, by the way... you can't take this car to Burning Man. If we find one speck of dust in the car, trunk, or engine compartment, we're charging you a mandatory $150 - $300 detailing fee." To which you will reply, "OUCH! I'VE BEEN UNPLEASANTLY BONED!"
5) You can't do anything about it at this point; your crap is in the car already, and all other rental car companies have the same policy. The rental company fully knows you're going to Burning Man... your fuzzy boot covers gave it away. ;-)
So, you have two choices:
 The Cash Option: Budget for an extra $150 - $300 in boning fees, because as soon as you return the car the first thing they'll do is pop the hood and you'll be found guilty. If you've got cash to spare, just pay the fee, smile and get on with your life.
 The Elbow Grease Option: Budget for 60-90 minutes of time and head over to The Wave Car Wash (see below)-- it's a DIY car wash that's open 24 x 7. Wash the ever-loving bejeezus out of your car. Make sure you clean the engine compartment, door jambs, trunk, floor mats, between your buttcheeks, everything. It takes 60-90 minutes. You can't do it faster. (Trust me, I've tried.)
Did I mention that you should wash your engine compartment? You should wash your engine compartment. Give it a healthy rinse -- don't use the high-pressure washer though.
Handy Boning-Avoidance Tips from Your Previously Boned Pal Jill-o:
1) Pack an easily-accessible garbage bag in your luggage. Before you leave the airport, take all of the floormats (front and back!) and put them in the garbage bag to keep them shiny clean. You will thank me.
2) Pack some glass cleaner and paper towels and leave it in the car. You'll need them when cleaning the car, and the car wash probably won't have 'em.
3) Bring many giant contractor bags and leave them in your car during Burning Man-- keep them clean! Before you put your crap back in the car as you pack up to leave the playa, put your dusty containers in the clean contractor bags to keep the dust inside the bags and out of your car. You'll thank me.
4) Hit the Gerlach High School Community car wash as you leave Gerlach. It's cheap, it makes you feel good, and it'll get Layer One Of Dust off your car. Note: This is not enough cleaning to avoid THE MANDATORY UNPLEASANT BONING, however.
5) When washing your car's dashboard/interior doors, remember that wet/moist dust is harder to remove than dry dust, so vacuum as much as you can before using anything moist, or you'll just smear mud around.
6) Having an OCD person in your car is helpful. ((giant poop-eating grin))
The Jill-o Approved Self-Serve Car Wash:
 The Wave Car Wash: 1201 E Prater Way In Sparks. (When coming home from the playa, take I-80 W, get off at the Sparks Blvd exit (Exit 20) and bang a right off the exit. Make a Left onto E Prater Way. It'll be on the corner of Prater Way and Parlanti Lane.) This place is handy because it's out of town and won't be crowded. They take $1 and $5 bills, Visa, Mastercard, and tokens.
 The Wave Car Wash also has other locations around Reno, too: http://www.wavecarwash.com/Content.cfm?CopyID=225 They're all open 24 x 7.
I hope this is helpful, and I hope it helps you avoid the UNPLEASANT BONING.