I'm a big fan of the digital hygrometer. I got one from Cigarmony, and it replaces the cheap-o analog unit that came with my humidor. (I have the round one shown here: http://www.cigarmony.com/index.asp?P...ATS&Category=7
The other thing I do, to keep my humidity constant, is to use Relative Humidity beads rather than any of the gels or PG solutions on the market. I keep my humi at 65%, it seems to be a perfect compromise (many fine cigars don't like 70%, and will develop mold) I went in with a few buddies, and we bought a pound of beads. You take the correct amount, stick them in a panty hose that I got from my wife, and it fits where the floral foam junk was in my humidor. I've now got an extremely consistent humidity, and an excellent gauge, with no loss of storage space in my humi. http://www.cigarmony.com/index.asp?P...ATS&Category=8
Someone here mentioned a Cigar Oasis. They're pretty awesome, but you need a larger humidor to justify one (3+ cubic feet of storage, or a fridge-a-dor) Sticking one in a 100 count humidor will take up a lot of space, and overpower it quickly.
Seasoning your new humidor can make or break its consistency. Read up on it. It can take a couple of weeks to do it right.