Just don't spend it, that's the key. ;)
A simple solution: auto-transfer the 'extra income' every month to a low-interest checking/savings account. Once a year, transfer some amount from low-interest savings into an index fund.
A simple solution: auto-transfer the 'extra income' every month to a low-interest checking/savings account. Once a year, transfer some amount from low-interest savings into an index fund.