I've had my current favorite motorcycle helmet for almost two year. It could do with freshening up some way. Luckily my helmet has removable and washable lining. But what's the best way to do this, right...
If your motorcycle helmet has a removable lining and cheek pads it's easy. Take them out carefully and wash them in a bucket by hand. Use lukewarm water and baby shampoo. Spend time squeezing the lining so the soap penetrates the foam cells fully.
Then rinse I. Again, spend plenty of time working and gently squeezing the lining to get all the detergent, dirt, sweat and bacteria out. Squeeze out as much water as possible without being too brutal, and hand the lining out to dry on the washing line. If your helmet has a fixed lining, it's still a fairly easy job. Most helmets with fixed linings can be washed, but check with your motorcycle helmet manufacturer just in case. Put the lid on a towel in the bath tube and follow the same procedure – work the baby shampoo into the lining and repeat with clean water to rinse thoroughly. Gentle use a toothbrush or cotton buds can help clean the outside and vents.
Place the helmet in front of a ventilator fan, with vents and visor open to dry it. It'll dry (more slowly) if placed upright in the airing cupboard but be very careful it doesn't get hot as that can cause damage.