When the sound is a long "e" (as in feed), write "i" before "e", except after "c". After "c" reverse the spelling ("ei"):

After other letters

believe, yield, reprieve

After c

ceiling, perceive, conceit

The problem with this rule is that it works only when "ei"/"ie" sounds like the "ee" in feet. If it has any other sound, you should write "ei" even after letters other than "c":

foreign, vein, freight