Soft latching power swtich example.
There is almost no current when the circuit is in "off" state (classic flip-flop with two NPN transistors draw current in every state).
It is without inrush current limiting, see my other circuits for inrush current protection and less losses in the transistor switch.
Resistor values are probably not the best, play with it.
If the resistor R4 is too big the latch is turned on by default and does not stay off when button Turn_OFF is released.
For better practical circuit search for this video:
EEVblog #262 - World's Simplest Soft Latching Power Switch Circuit