A common term for what you need is a bleeder circuit, and the simplest form is a bleeder resistor.
http://en.wikipedia.org/wiki/Bleeder_resistorThe principle is in the schematic. Naturally add support or protective circuitry as needed depending on the actual voltage or power involved in your PSU.
In bleeder1, RLY1 is powered before the regulator circuits. At
X, R1C1 may be before the regulator circuits, thus shorting out the main filter capacitors, or it can be after the regulator circuits, discharging regulated DC output capacitors.
In circuit bleeder2, an AC relay is used instead.
Its a good ideal to have an LED or any annunciator reflect the status of the DC side to insure its discharged in case of failure of R1 or the relay.
One advantage of old style analog meters in PSUs is the meter is powered by the output port. So unlike a digital panel meter, you know the status of your output without question. Analog meters are very reliable, and if they did fail, its easier to detect than a simple off/on LED and nothing can be gleaned once a digital panel meter is powered off even if the PSU outputs remained energized.