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.
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When my tobacco is dry i spray some water in the pouch and mix it up.
It works perfectly fine for me.
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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.
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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.
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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.
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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) :)
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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
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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.
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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.
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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.
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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.
How to keep tobacco moist?
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There are a few different ways to keep tobacco moist, and most involve keeping it in a zippered plastic bag. Inside the bag, you could also keep a slide of bread, a damp sponge or moist cotton ball to provide moisture. You could also spray some water in the bag when the tobacco gets dry. Apple slices or potato peels can also be effective. The options include a zippered bag with a slice of bread, spraying water into the pouch when dry, placing moist cotton balls, cigarette filters apple slices, pieces of terracotta, cedar wood potato skins, a Cos lettuce leaf or a damp sponge or into the pouch.
Zippered plastic bag + slice of bread which should be changed every few days. Spray water in the pouch when dry. Put a moist cotton ball or cigarette filter in the bag. Apple slices keep the tobacco moist and taste good. Add a piece of terra cotta soaked in water. Types of wood such as cedar or a special pouch humidifier. Thick cut potato skins. Use a zip bag and keep a damp sponge inside. A leaf of Cos or Romaine lettuce.