Whether you're a new or experienced rider, chances are you're going to have a smelly/dirty helmet after all the motorcycle riding you've done. A lotta people ask how to clean lids inside and out, and although there are a lotta different methods and suggestions, I've compiled the most effective ways to do so. (Plus there wasn't a how to on this anyway, so I hope it helps)
If you own a helmet with a removable interior lining, then you pretty much have it made. You remove the lining and/or the cheekpads and throw them in the laundry machine at delicate cycle, while that is washing, you can clean the outside of the helmet with a soft cloth/towel/micro fiber and mild detergent. Moisten the soft cloth/towel/micro fiber, put a few drops of mild detergent on it, and wipe the shell gently, the detergent will do it's job with the grime/dirt/etc and after you're done, you simply wipe off with water until the detergent is gone. You'll be left with a shiny clean OIL FREE shell.
BUT for those of us with helmets that do NOT have a removable liner.. the job is a little trickier. Skin oil and sweat builds up and all the "spray cleaners" just don't quite cut it. I've tried helmet fresh and all it did was make it smell better, and I've tried Motorex Helmet Care Spray, which yielded same results, all fragrance, minimal cleaning effect (on the lining).
Follow these steps if your helmet doesn't have a removable liner and smells like poop.
Buy a helmet to clean if you don't already own one unless you want a how to: stitch your own head/scalp/face up thread.
Remove the cheekpads, breath guard, visor, etc.
Grab some shampoo. If it's good enough for your head, it's good enough for your helmet. Dish detergent might be a bit harsh for the interior, so avoid that.
Don't overdo it, but don't be cheap with it. If your helmet REALLY smells, then go ahead and put some more.
Fill with lukewarm water. I have a feeling cold and hot might be a bad idea, stay away from the extremes.
Rinse to agitate the excessive dust/dirt out, you don't want to dunk it in the CLEAN water and scrub with the same contaminated water.
NOW dunk it in and douse that mofo in cleanliness.
Get it all up in there.
Gently massage the lining so the shampoo works its magic.
Rinse out all the shampoo thoroughly.
Pat dry the exterior to avoid water spots and let it air dry. If you live someplace humid or extremely hot, you may want to place it in a cool DRY place with a fan blowing it or something. Look at the shine!