The cause of this is the design of the electronics inside the bulb combined with stray capacitance between live and switched live. In the UK it is most commonly seen on 2-way switching installations due to the physical conductor layout in the cable used for those.
Basically the circuitry inside the bulb contains a rectifier followed by a capacitor, followed by some electronics that has a minimum voltage threshold. The capacitor charges (through the stray capacitance) until it reaches some threshold where the electronics in the bulb try to start up producing a flicker of light and discharging the capacitor. This process repeats.
One fix is to add a a capacitor in parallel with the bulb. This forms a voltage divider with the stray capacitance preventing the voltage build-up. (for those wondering why this is different from the capacitance inside the bulb it is because it is before the rectification while the capacitance in the bulb is after it)
In theory to calculate a minimum value for the capacitor you would need to know
- How low the voltage needs to be held to stop the LED bulb flickering.
- What the stray capacitance is between live and switched live.
Unfortunately neither of these values is readilly available so it becomes a case of rules of thumb rather than an exact science.
The capacitor needs to be of a type suitable for connecting directly across the mains. It's usually also good if it has some resistance in series to limit current surges at switch-on.
One forum thread I found suggested http://uk.rs-online.com/web/p/rc-network-capacitors/2067869/ which seems fairly reasonable to me.
The capacitor needs to be mounted inside a suitable electrical enclosure.