I have heard of lemon, apple, cucumber, copper coins, steam and read about freezing, storing in airtight containers. I have tried cucumber and I can't say it really worked.
Knowing the power of stackexchange I thought I would try and dig out the ultimate way to keep tobacco, in either a tin or pouch, moist.
Zippered plastic bag + slice of bread. The bread will slowly provide moisture to the tobacco and help it stay moist. Change bread as needed as it will become hard within a few days. This trick also works for cookie jars.
When my tobacco is dry i spray some water in the pouch and mix it up. It works perfectly fine for me.
I've used a moist cotton ball or moist cigarette filter (unused) in the pouch with good results. I've also breathed into the pouch through my mouth for 20-30 seconds, closed the pouch tightly and let it sit for a while unopened.
My personal experience and from that of the old timers is apple slices. Not only does it keep the tobacco moist, it adds a taste that pairs perfectly with tobacco.
You need to use a piece of terra cotta. They sell them on amazon, etc. for use in brown sugar. When you get it, let it soak in water for about 30 minutes. Then pat the surface dry and throw it in with your tobacco (or "cilantro") and it should keep things moist for about 3 months.
You can use certain types of wood. I think cedar is recommended.
Native American Spirit's has a pouch humidifier for their roll your own tobacco pouches. (Sold Seperately) :)
thick cut potato skins work very well, as they don't leave behind any scent or taste that could contaminate your tobacco being a quite neutral earthy vegetable, and can keep it moist for up 3-4 days.
further to @ adems answer I have also sprayed my tobacco to rejuvenate it before now, but id spread it out and use something that mists rather than sprays as I have also ruined a pouch this way (you could dry it out again but it becomes very weak tasting) hope this helps
Zip bag + sponge.
Wet the sponge, squeeze it. Put into bag. Close bag.
Voila. Should hold for quite some time (At least 3-4 weeks if not more)
You can cut off piece of the sponge to make it smaller.
Lettuce leaf inside the pouch (either leather with a zip or the plastic wallet you buy it in, keeps moist better than a tin) - will need to be replaced every 2 or 3 days, depending on its size and type of lettuce. Cos or Romaine lettuce, followed by Little Gem, is the best, because they have a thick moisture filled spine on the leaf - break the spine a bit before putting in the tobacco.
I'm mystified though - I like my tobacco fairly dry, so I only resort to lettuce when the weather is very hot and the tobacco's become crispy. I've spent more time drying out my tobacco (sometimes in a warm oven) than keeping it moist over the years...but then the UK has high humidity most of the time.
We use a sponge in a ziploc bag which is left open, and the sponge saturated and this works fine with our tobacco to keep it moist.
