I've given up the stand-by travel thing. First of all, it ain't that cheap anymore, you're less likely to get an upgrade, and it's just far too much hassle. However, based upon many years of stand-by travel experience I'd say there's no hard and fast rule. Generally, it's a good idea to try and avoid peak days/times. That may be a little difficult on LHR-LAX route in August mind. Some carriers are not in too much of a hurry to reimburse unused s/a tickets and some may impose an admin charge. Go for carriers with the highest frequency of flights. Aim for midweek flights (Tuesdays can be good) and aim for the first flight of the day. Pax may miss connections and not show, but these will accumulate over the day and fill later flights.