How do I choose a ferite bead for power supply filtering?
It seems like you are concerned with the output currents from a SMPS that supplies a mixed signal system.  The most important thing will be to look at the spectrum of the of the current from the SMPS to find out where the energy is.  This will include differential and common mode sources.  

For a SMPS, operating at a PWM frequency less than about 200kHz for example, there is usually significant spectral energy from about 1MHz to about 10MHz.  So, that frequency range often needs added filtering.  But, it depends on PWM frequency, topology, and circuit parasitics ... so you will have to do your own calculation/measurement to see where the energy spectrum is for your case.

A lot of the beads that are commonly available (a lot of what you will find at digikey for eample) use magnetic materials that are not very effective (don't have much impedance) in the 1 to 10 MHz range, and don't become effective until frequencies reach 100MHz or higher.  The lower frequency stuff is out there, but you have to look carefully for the right frequency band.  

If you have an isolated SMPS, where you have access to the positive and return lines, a common-mode choke (CMC) would work better than a bead in the 1 to 10MHz range.  For either a bead or a CMC, filter performance can be improved a lot by putting small high frequency shunt capacitor(s) between the SMPS output and bead or CMC.  

Once you have identified the frequency band and attenuation you need to get down to a defined amount of noise, you can worry about current rating and DC resistance.