If you are hooking up a switch of some kind, you can eaily make use of the internal pull up resistor. Usually we use a switch to turn on the electricity. You push the button and the line goes high. and you would need a pull down resistor to tie the input low. But we also make the electricity go the other way. You use the pull up resistor to tie the input high. You hook one end of the switch to the input and the other end to ground, then when you push the switch the input goes low. Then in your software you look for the input to go low when the button is pushed versus high.
from this thread: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1205614794