Check the official wish list first
CPD added a wish-list feature this year that lets you flag programs before registration day. Worth doing even after the fact, since it's the fastest way to see status changes.
Understand why spots reopen
Families change plans constantly between registration day and the actual session: a work schedule shift, a different camp working out, a sibling's schedule changing. Every one of those is a canceled spot going back into the pool, and it can happen any day, any hour, all season long.
Check back at specific times, not randomly
If you're doing this manually, mornings and Sunday evenings tend to be when cancellations from the week get processed.
Try the same program at a nearby park
Multiple park locations often run the identical class, and a location with a longer commute may still have room.
Call the specific park
Front desk staff sometimes know about a cancellation before it reflects online.
Automate it
This is the part almost nobody has time for: refreshing a page for weeks on the off chance something opens. That's the actual gap ParkDibs (parkdibs.app) exists to close. You pick the program, it watches for a canceled spot and alerts you the moment one opens, so you can register yourself on the official CPD site before anyone else even sees it. Free to try, and free entirely for families on CPD financial assistance.