From Philip Marshall

This mod should work for just about any pedal, but I did it on my big muff,
and it was suprisingly easy (I only ended up adding one part!)

Now, the op-amp big muff already has a 220uF cap from positive to negative
(The transistor version of the big muff only has a 100uF cap, so you may
want to put another 100uF cap in parallel with it to get 200uF. For other
pedals that don't have caps directly from +V to GND, try a 220uF or 330uF
cap. That should be enough for the current requirement of most FX). The
problem is that point A and B must always be at the same voltage since
there's no resistance between them. So, the capacitor can't filter out the
ripple. The simplest things to do is to add a resistor between point A and
B or the order of one or two hundred ohms. This way, the capacitor can stay
at a constant voltage while Vin varies. I tried 100 and 330, and it sounded
better with 330 ohms.

The hum is gone! This circuit works fine, but now there's a voltage
difference between A and B, which effectively means that the pedal is now
seeing less than 9 volts. For my pedal, with a 330ohm resistor there was a
drop of ~1V. I decided that wasn't good enough.

Inductors act as short circuits for the DC components of voltages, but act
as impedences for higher frequency signals (the impedence is directly
proportional to the frequency). So, DC current can flow without causing a
voltage drop accross the inductor, but the AC current is limited. Now,
points A and B can be at the same /DC/ voltage, but the voltage accross the
capacitor can still remain constant. I used one of those little axial
inductors that look like resistors, I doubt you'd actually want to use a
coil.

This sounded even better. I measured a voltage drop of ~0.02 mV accross the
inductor with my DMM. You may be able to use smaller valued inductors, but
I didn't want to buy any just to experiment. Even if your pedal doesn't
have caps, just adding an inductor in series will help smooth out the
voltage, and you might find that you don't need a cap.