Storing a counter to EEPROM - how to extend the life?
If possible use a combination of wear-leveling like algorithm (as pointed out by Mikael Patel) and an power-down driven interrupt, to save onto EEPROM only when needed. At reset, load the last keypress count into a variable in RAM. Increment that variable in RAM on keypress, and save it to EEPROM only when a power down occurs (i.e. the interrupt is triggered). Optionally, when power down occurs, make sure to save the variable only if the new counter actually differs from the old value (i.e. if there actually were keypresses). 

To implement the power-down driven interrupt use the built-in comparator to monitor a Vin (use a resistor divider!) against the internal Vref, and use the analog comparator interrupt to save the value. Remember to put enough capacitance between +5V and gnd, to make sure your arduino has enough time to save the state.

Alternatively, if you can afford errors (which might accumulate over time, and lead to a huge underestimation), save the value to EEPROM only every n keypresses.