Tatami mats

How do you clean tatami mats?

You wipe them down with warm water and a gentle soap. theyre very durable

Just make sure you don't leave the tatami mats wet/damp; if you wipe them down with water and mild soap, be sure to wipe them dry with a piece of dry cloth afterwards, or air the room completely to let the mats dry. They don't like moisture.
In the first place, vacuuming it (along the straw, never across) will keep it fairly clean too :)

It depends on what you want to clean off. If there are no obvious dirt or stains, all you have to do is to vacume or broom regularly. You need to exchange mats once in a few years, though. Also, don't keep your futon laid on the tatami at all times.

I just wanted to know wether to sweep or wipe the mats. I ate in the room and there are a few crumbs. Thank you for all the answers, I will be careful when I vacuum that room.

Marcello, if they're not oily or moist crumbs I would just use a vacume cleaner. If you are worried about oil or any kind of stains or stickiness, after vacume-cleaning, I would put a bit of dish-washing soap on a cloth and wipe it. Then, as mentioned, be sure to finally wipe it with a dry cloth.

When vacuming, sweeping or wiping tatami, be sure to do it in the direction of the way the grass is woven.