pullup and pulldown resistor help

[sayzer]

Hi -

How do you know when to use a pullup or a pulldown resistor?
Know when to use what resistor and then what value is not straight forward to me and trying to figure out a simple way to remember to know which to use.

Thanks -

If there is a "memory save" button in the system, then I use pull down resistors instead of pull ups. => Button press pulls up.
Reason: If power is removed, then there is a possibility that pin goes down before PIC goes off. The program then acts as if the button was pressed.