How to select appropriate ferrite beads for power supply rails
Ferrite beads tend to work effectively in the resistive region of their frequency response. Take a look at this: -



Sub 1 MHz to about 10 MHz the bead is acting as an inductor so, if you plan to use a bead as a filter then you need to have a capacitor to ground on the output side (quiet side) to form a low pass filter BUT beware, if you form a resonant low pass filter with a Q factor that is too high you can get severe ringing on the output (due to fast edges on the input noisy side) and destroy chips on the quiet side. Been there etc..

Between  about 10 MHz and 300 MHz the inductance drops off rapidly (due to the ferrite material properties) and the bead looks resistive - broadly speaking, this is the target area that is best suited to ferrite beads because they really do dissipate the energy contained in noise that would otherwise have plagued your quiet circuitry. You still need an output capacitor in most cases though.

Above 300 MHz the bead is starting to act as a capacitor and progressively becomes useless.


  How to select appropriate ferrite beads for power supply rails


Decide on what interfering frequencies you generally have. Decide on which of these interfering frequencies will give you problems. Choose the most appropriate bead or other method.