whats the best way ??

Carefully.

-- Fermez le Bush

Perhaps with a separate counter. Then just read the value of the count when desired. Many micro's that I've seen include various commonly needed 'extras' such as this. If the signal is 'noisy' you may need to condition the pulses somewhat before putting them to the clock input of a counter (schmidt trigger comes to mind as one way, or a one-shot circuit to limit the upper rate of triggering).

Some 'one-wire' devices do this and store the results internally. When queried (over the serial line), they respond with a hex value.

daestrom