Once external temp exceeds internal temp, (about 11 am, recently) shut windows and close blinds/curtains/shutters.
Use fans if needed, and avoid upstairs rooms if possible.
Once external temps are below internal temps (about now, here in Welsh Marches, UK)
Open upstairs windows, put an extract fan upstairs pointing out of the window (this draws cool air into the house and blows hot air out)
Keep internal doors open to encourage cool air flow