There are a few options to increase humidity. Some were already great suggestions.
1) cover the tank lid w/ tin foil, plexiglass, or a damp towel
2) you can mist manually with a spray bottle
3) get a larger water dish
4) put the water dish on the hot side of the enclosure to promote evaporation.
And a thermostat is a must have when using a heating pad. You can't have a heating pad w/o a thermostat. A heating pad can exceed 130+ degrees when unregulated and can burn your snake.
Here are a some good thermostat recommendations:
The Herpstats are top notch. They are very accurate, safe, and reliable. They are expensive, but they do their jobs very well. The base model starts off at $99
http://spyderrobotics.com/index.html
If you're on a budget, the $27 Hydrofarm from Amazon(w/ free shipping) is one of the better lower end thermostats. Better than any petstore thermostat in my opinion. It's not as reliable or accurate as a herpstat, but it works fine.
http://www.amazon.com/Hydrofarm-MTPR...7876947&sr=1-1