I use a common household canister filter. Filter setup
I clean and sanitize a keg, leaving the sanitizer in the keg. I pressurize the keg to about 5 psi and connect the filter to the OUT. This lets the sanitizer flow through the filter, then the CO2 blows the sanitizer out of the filter. I've found this works better if you turn the filter upside down.
Once the filter has been blown out, I disconnect it from the new keg and connect it to the OUT of the keg with the ale in it and the IN of the freshly sanitized keg. I use about 5 psi on the source keg to push the ale through the filter. You have to burp the target keg by pulling the ring on the pressure relief valve once in a while to keep the ale flowing. Setting the pressure relief valve so it is just out-gasses is a good way to spray foam all over the garage.
Once the source keg is empty, I invert the filter to get the last quart out of the filter into the target keg. If I was starting over, I'd get a 5" filter rather than the 10" one, less ale loss. Most of the time the 0.5 micron filters do the job.
DO NOT USE A CARBON/ACTIVATED CANISTER! These are sold for cleaning water and they'll do a real number on ale!
I probably filter 10-15% of the lighter ales. Most dark ales, I don't worry about.