Have NC for this as I don't want to out myself.
Mostly, we live below our means. We now have no debt other than mortgage and a credit card that is paid off monthly, run one (old!) car, have one foreign holiday a year where we travel to stay with family and one Sun holiday short break. We don't go out much (we have smallish kids and no family support/convenient babysitters local to us) and neither of us spend loads on "stuff".
I'm signed up to Quidco, Yougov and a couple of other survey sites, and the Mumsnet panel :o which has been quite good to me over the past year!
We track income/expenditure across all accounts (share dealing, ISAs, kids savings, estimated house price, mortgage balance etc) on a spreadsheet so we can see our total net worth fairly easily. In the past we tracked spending for a month, then used that to create a sustainable budget. We both earned fairly well and lived off one salary so saved the other (practise for having kids, as we intended that I would be a SAHM and wanted to be sure we could afford it). I walked away from my last job with about a year's salary after signing a compromise agreement - not something that will happen to many people, I grant you!
Currently, neither of us is working and this is sustainable indefinitely because we have invested enough in shares to be able to pay all our outgoings from guaranteed dividend income. It does mean we can't easily change our mortgage provider, although we're on a pretty good rate so it's not a concern in the short/medium term.
Mostly, genuinely, we got lucky - we invested in some shares that over the next 3 years or so, increased in value by 10x what we paid for them. We sold them, and are where we are. We are not increasing our investments, but when the kids are older and I have some time to find a part time job, and my husband goes back to work (planned in around 3 years time) then we will.