Use half a box of baking soda with a cup of warm vinegar. let this work for an hour or two, then pour down a lot of very hot water. An alternative is to use equal measures of baking soda and table salt, washed down with boiling water.
White or Applecider vinegar work equally well.
Editor: I normally use a few heaping tablespoons for most clogged drains. This is also a useful maintenance do to bathroom and kitchen sinks plus shower/tub drains a couple times a year to just keep them cleaned out.