Joule thieves are getting more and more popular. A typical BJT-based joule thief has some drawbacks from its design. Namely, the input voltage must be at least 0.7V to overcome the base-emitter resistor of the transistor. Moreover, the output of the circuit will vary depending on the input voltage.
Here is a design idea from EDN utilizing the benefits of MOSFET transistors
This design is a little bit more complicated. It uses a low threshold MOSFET and two coupled coils to operate as a joule thief. A second MOSFET is used to regulate the output to the desired voltage.
Here is the link with the complete documentation of this circuit.