how do you keep a constant humidity?

[Flock Runner]

i want to start hatching some of my eggs. i finally think i saved up enough money for an incubator and an egg turner but how do i keep the humidity at the rite level? i herd somthing about putting a glass of water in there if its too low but what if its too high?or is there an automatic device that will keep it level for you? i'm not so lazy that i NEED technology but i don't want to end having half of them not hatch and the others hatch deformed.