Do you need to humidify your Upright Bass?
If you're after one of those rubber thingies to hang in your bass to keep up the humidity, I have them. They consist of a perforated non-vinyl tube with a sponge material inside, with a disk on the end. You soak it in water, squeeze out the excess, wipe it off, and hang it inside your bass though an f-hole. Put the bass into a bag/case, and natural evaporation transfers the moisture into the inside of the bass. (If your bass is out in the open, the moisture just evaporates into the room, having very little effect on the bass).
But the question is... do you really need one?
If you have a laminated (plywood) bass, no. Carved or hybrid (carved top), yes, or at least, maybe.
The whole idea really applies only to carved basses, whose massive tops and backs can shrink or swell significantly when humidity drops or increases beyond the usual environment. If my old carved Juzek bass is used to normal or average summer humidity here in Southern New Jersey, when winter comes and my forced air heat comes on, the mental alarm bell goes off. Indoor humidity drops like a rock, causing moisture to gradually leach out of the wood, causing that top or back to shrink -- that puts pressure on the whole structure. If I'm lucky (and my bass seams are glued properly), the thinner mix of hide glue at the seam lets loose-- sort of an organic safety valve. The fix for that is usually an inexpensive one.
However, if I'm not lucky, "Crack!" goes the bass. "Rats!" goes Bob. Bob goes to the luthier and pays him potentially very big bucks to repair that ugly crack, which will haunt him forever. Bob says "Rats!" (or another, more colorful phrase) again.
Laminated (plywood) basses were developed for this very reason. Their more stable woods don't move with the weather enough to be concerned, since they are not solid wood; so you don't expect them to crack under those circumstances. That said, extreme conditions -- and especially, rapid CHANGES between conditions -- aren't good for any instrument. Very high heat and humidity can affect the hide glue, causing seams to separate, and it is possible that extremely dry weather could affect the typically solid wood end-block, though it doesn't happen often.
These little hose doohickeys (Dampits, RDM Humiditrons, and generic versions) can be a mixed blessing. First, you have to constantly watch them and remember when you last dunked it, and unless you have an accurate gauge, you really don't know how dry the house's environment is. Secondly, sticking one in your uncovered bass in an otherwise dry room or house doesn't help much - as the moisture evaporates out of the tube, it humidifies the air throughout the entire room, not just inside your bass. So it really is only worth using if your bass is inside its case, and therefore in a closed environment.
So they have their uses and can be effective if used faithfully; however, some luthiers smile when they get to charge the overly faithful for cleaning the mold out of the inside of too-humid basses when overenthusiastic users don't exercise enough care. They are not necessarily a bad idea as an on-the-road precaution if the bass is out and about for a long while, but don't worry about taking your bass to a gig without one, it shouldn't be a problem over that short exposure - unless you're playing an outdoor gig at the Arctic Circle. But it could be a great conversation piece when your fellow players see it and ask, "What's that??".
If you've got a carved or hybrid bass, my suggestion is to get a household humidifier at your local "X"-Mart or equivalent; preferably one with a built-in humidistat control. I've got a couple different models that I picked up for around $50-60 each, and keep them where I store my basses. I also have one in my bedroom to keep my pipes moist (well, my throat, I don't have pipes any more). Both of my humidifiers have digital readouts that show the humidity in the room and let me set a target number. The units automatically go on and off all by itself to maintain the setting (I go for between 30% and 40%.) IME it's better to get one with a humidistat with a digital display - those I've bought with dials don't seem to have precise enough sensitivity. I have had a couple different humidifiers in our large home living space, where I have an electronic hygrometer to monitor the humidity, and on/off cycles can vary by 15% or more. Spend the extra bucks and get a good one, keep it clean and it'll last several seasons. If you do just get/have a regular humidifier without a digital control, you'll find little electronic thermometer/hygrometer units that typically have a probe to measure outside temperature as well as indoor temp and humidity.
All I need to do is remember to refill it, and their angry little flashing lights as well as a low humidity number reminds me. It's a great excuse to keep your bass safe in your bedroom, because you then can humidify it and your lungs at night and not wake up with that dry and scratchy throat on those brutal winter mornings. At least that's what I told my wife. Just a note-- if you are going to put one in your bedroom, don't get a cold water evaporative unit, as the fan going on and off at night can disturb your sleep. The hot steam and ultrasonic types are usually relatively quiet.
We also have several tools for humidity and temperature monitoring - worth a look, to protect your investment!
Now go forth and humidify! (IF you need to)
I should mention that ALL humidifiers need maintenance; leaving them to gunk up means you could have problems later on. I have had one hot steam humidifier for about seven years and it's still working.