Zener diodes try to keep their terminal voltage at their designed voltage, that is their function. Putting a zener diode in parallel with a load can help regulate the voltage to the desired amount, if the load is fairly small (you are using very little current). Exact values would need to be calculated - i'd recommend using a simulation program like PSPICE (free student version - search the web) to get an idea. If more power is needed, a simple power supply design would need to be used - a zener diode can be used to the gate of a power transistor to force the gate voltage to stay the relatively constant. Wire the collector of a NPN type to your rectified source, and the emitter is your positive output. The zener diode will keep the gate voltage relatively unchanged, keeping the emitter voltage relatively constant. A fairly large capacitance on the output of your rectifier circuit will help remove a lot of the ripple; the transistor + zener will clean it up further.