Most programmable lighting desks have an effects unit with random and
regular flicker effects.

If you don't have an effects unit and you need a random flicker you can
try the classic fluorescent starter trick. Wire a bulb in series with a
fluorescent starter. It will flicker on and off at random, slowly for
the first 5 or 10 seconds, then a little quicker.It's worth buying the
proper bases for the fluoro starters, as their terminals don't like
solder! Note that the starters have a maximum rating of 40 or 60 Watts.

The on-off random flicker from a fluoro starter is a bit unsubtle on
it's own. Pair the flickering bulb with a bulb that's permanently on for
the best effect. (Steady blue and flickering white works well for
television, steady red and flickering yellow for a fire).

For a really top-notch fireplace, use a red bulb non flickering (but
possibly on a slow fading sequence) plus two flickering bulbs, one in
orange and one in yellow.

See 'flickering candles' under the /props/ section of the FAQ for
flicker dimmers and flickering bulbs.