Not sure if it's the same on all servers, but this is how I do my cron:
Minute: 45,0,15,30
Hour: *
Day of the Month: *
Month: *
Day of the week: *
Command: GET
http://www.collegepranks.co.uk/forums/signature.php >/dev/null 2>&1
Obviously the 'GET http://www.collegepranks.co.uk/forums/signature.php' part is to run it, but the '>/dev/null 2>&1' part stops me from getting a report every time the cron is executed.