It is well known that 3D printer filament is susceptible to moisture, some materials more than others. When you receive a new roll, it's vacuum sealed in plastic. After opening, one can either:

Use up as soon as possible
Store in an area or container with low humidity
Reseal in some way
"Dry" out before using again

As a hobbyist, I can't use a whole reel quickly. Furthermore, I tend to have a fairly large number of reels so I can print in different colors and materials. So, some reels may go unused for a long time. My home is air conditioned which helps reduce humidity. I also store my reels in a large, home-built, "semi-sealed" cabinet with desiccant. But, the humidity inside the cabinet is essentially the same as my home.

Mostly, I use ABS filament and have not really had much of a moisture problem. However, I sometimes use Nylon and it is like a sponge. You can easy tell if your filament is wet during printing:

You see "wisps" of steam coming from the extruder
You hear "cracking" and "popping" as the filament extudes
You see "bubbles" in the extruded filament
The surface of your print has extra imperfections (artifacts caused by bubbles)

I recently began re-vacuum sealing reels in FoodSaver bags as shown in the picture.

Step 1: What You Need

You need a FoodSaver thermal sealer, or equivalent, and FoodSaver bags, or equivalent. Ebay has a large number of bag suppliers. I have used this one for both kitchen and 3D filament sealing:

Some Notes:

If you already own a FoodSaver machine, it should be adequate. We had replaced our older machine with a newer version so I use the older one. Machines are available in different sizes with different "bells and whistles". All you really need is one that will vacuum out and then seal. The first photo shows our sealer. While you can cut the bag rolls to size with a pair of scissors, a machine with a built in cutter simplifies cutting. The second photo shows the bag cutting feature. Usually, machines with bag cutters also have room to store a bag roll. However, the roll of bags I purchased was too large to store in the machine. Not a real problem.
You need a FoodSaver brand or equivalent bag roll or "pre-made" bags. Pre-made bags come in a fixed length with one end already sealed. I choose to make my own bags from a roll so that I could cut to whatever length I needed and purchased a 50 foot long, 11 inch wide roll from Ebay. The maximum bag/roll width seems to be 11 inches, probably due to sealing machine width limitations. 11 inches is a somewhat "tight" fit for a 2 pound reel, but not so tight as to not work just fine. A bag length of about 16 inches is adequate, but, if you make it longer you can re-use and reseal a bag after opening.
When sealing, you will note, and possibly see from the photo, that the center (hub) area recesses into the hub as a vacuum is pulled. Depending upon the amount of vacuum from your machine and the thickness (quality) of the bag, this area may "pop". I haven't had this problem, but, one solution would be to put a piece of cardboard or other material over the hub hole area to reduce the stress in this area.

Alternative:

Before I started using the FoodSaver method, I tried sealing reels in "Space Bags". The first problem was that I couldn't get bags that were small enough. This made storing the sealed bags more difficult. I also tried Space Bag "lookalikes" from the Dollar Store. I found that these didn't hold a seal very long. Another problem with this method is that you have to drag out the old Hoover every time you want to seal, maybe more difficult that using the FoodSaver.