Feel like checking in to remember the day but don't feel like sharing it with everyone? To hide a check-in from your Foursquare friends but still have it show up (just to you) in your personal check-in history, tap the icon that looks like a group of friends so that is changes to a 'lock' on the side of the check-in screen before you hit 'check in.' Once you want to check in publicly again, tap the lock button until it looks like a group of friends.
Private check-ins will not be visible to the public or your foursquare friends. These check-ins do not assist in earning mayorships nor badges. To view these check-ins, simply navigate to your history page.
Note: private check-ins cannot be turned into public check-ins after the fact, nor vice versa.