For us it will have to be a combination of things but mostly it will be done using real estate.

IMHO Mortgages don't need to be paid off, we just need to collect more rent than the mortgage repayment.
Over time the rents will increase (keeping up with inflation) but the mortgage repayments remain the same so one day the rent will more than cover the repayment and you'll start making money.

If you have secured a number of such properties you can let them work for you in the background until such time when they provide sufficient income for you to retire.
This is our plan anyway and we hope to retire at 55 which is not very early but it's better than the 70 years our governments have in mind....