How to change the Write Ahead Log directory
This can be done according to the documentation [here]:(http://www.postgresql.org/docs/9.1/static/wal-internals.html)


  It is advantageous if the log is located on a different disk from the
  main database files. This can be achieved by moving the pg_xlog
  directory to another location (while the server is shut down, of
  course) and creating a symbolic link from the original location in the
  main data directory to the new location.