Old money, going back several generations. Not necessarily cash rich though, some of the poshest people I've met have been quite skint when it comes to ready money.
A hereditary title in the lineage.
Staff.
Old houses, old cars, old clothes, old furniture, old art, old books. Old gundogs and old guns!
Male children going to the same public school for a few generations, usually followed by Oxbridge or the Royal Ag. Art college for the eccentric ones, army for the thickos. These days, I would expect a decent public school for the girls, too, but in my day (I'm 65) a good day school was considered adequate for daughters. 
A house in town (or a flat, times are hard for the gentry) and a house in the country.
And that air of utterly assured confidence and being at ease in any situation.