1 Press reset button and power button together
2 After about 5 secs release reset button, but keep pressing power button
3 When booting screen appears release power button.
4 It boots into recovery.
--Update 1-- some units without front USB have a different procedure
Firstly power on the unit and then long press the power button and then reset. Then the unit will get three rings and you loose your hand on the power button and short press the power button again. (thanx @netay)
--Update 2-- some others have this procedure (thanx @R4D3N)
1. Turn off the unit by holding 3-4 sec the power button.
2. Hold the RESET button + POWER button for 5 sec
3. Release the RESET button and keep pressing the POWER button for 2 sec
4. Release the POWER button and wait. The Recovery menu appears.
5. With the Volume knob you can move up/down and long press on Volume/Power button to select the option.