Signal strength on Freetronics 100% Arduino Compatible Board
When my digital signals fluctuate when I expect them to be a constant 1 or 0,
it's usually something about the pull-up resistors -- I haven't connected them properly, or they've somehow gotten disconnected.

Once I've ruled out that problem, my next most common problem is contact bounce.
Jack Ganssle ran some experiments and has some debounce recommendations.