Build a sufficiently large investment portfolio such that annual income from investments approximately equals annual expenses. Note that this may require moving to an area with lower cost of living (eg. working in SFBA for 10 years and then moving to the midwest).

The approximate rule of thumb (not accounting for investment variability, so be careful!) is that you can extract 4% of your investments annually without affecting the basis (while adjusting it for inflation). So if you can live off that 4% (ie. $40k off a $1M investment portfolio), then you can "soft retire."

There are entire communities dedicated to this notion (eg. http://www.mrmoneymustache.com/2012/01/13/the-shockingly-sim...)