I use Peterson discs, aka moisture pouch buttons, aka button humidifiers. It can be a slow process, but slow is best in this context. I've also used the damp paper towel method with great success. I prefer the discs, though. One disc for a small alteration. Two for the bigger jobs.
Mr.GL Pease wrote an interesting, more scientific post or blog on this. I can't remember where I ran into it, but at his website, in the FAQs, you'll find these Q/A:
Q: How much water is in there?
A: At 10% water (by weight), a tobacco is going to seem very dry. If the moisture level is increased to 20%, it will be quite damp. Ideally, moisture contents between 13% and 18% are right for most blends and most smokers. Some heavily sauced aromatic tobaccos are reputed to have non-tobacco content that approaches 40%. No wonder some of these seem so goopy! Not all that moisture is water. Various humectants (humidifying agents) are used by some manufacturers to preserve moisture levels at the desired percentage.
Q: How can I tell what the moisture content is?
There are destructive methods to measure it accurately, but it isn't really necessary to know the precise moisture content unless you're just the curious sort. As mentioned above, at about 10% and below, tobacco will be quite dry feeling, and the strands will tend to break when handled. At about 12-13%, the strands will be pliable, and will endure more vigorous handling without damage. If you press the tobacco into a ball, and it stays compressed, it's over 18-20% - too moist for proper smoking. In the 15-18% range, the ball will be springy. Once you find your preferred moisture, you'll be able to tell by feel whether it's there or not.
Q: What should I do about a tobacco that's too dry?
A: My method is to put the tobacco in a large, clean bowl, and cover the bowl with a damp towel. The towel should not touch the tobacco. Check the tobacco every couple hours, and when it reaches the moisture level you like, store it in an airtight container. Glass "bail top" jars work well, but be sure to clean them thoroughly. (See the next item on mold.)
Different tobaccos take up moisture at different rates. The denser leaf, like Virginias and some orientals, take up moisture very slowly. Spraying with water is dangerous, since it's difficult to control the overall moisture level of the tobacco. It's hard to evaluate the difference between damp leaf and soggy leaf. The method outlined above is pretty much foolproof.