I would call the morning of, let them know your intentions of taking a later flight (and which one as they can give you your odds of making it). They usually confirm that you want to standby on a later flight and will cancel your existing leg to avoid a 'no-show.' I've been pretty fortunate getting on later flights with my plat status (knock on wood..) In regards to going to the airport, the earlier the better. Ex. I went to SAN 4-5 hours before the flight I intended on standing by for just to get on the standby list and the standby 'ticket.' I then went about my business and came back 45 minutes prior to my flight (standby). You can't be placed on the list over the phone.. Has to be done at the airport (in my experience anyway)