I use perl for this as well.

  perl -e 'chdir "/var/session" or die; opendir D, ".";
      while ($f = readdir D) { unlink $f }'

It is very efficient memory-wise compared to the other options as well as being much faster. It is also easy to apply filters as you would with -mtime or such in find, just change the end statement to:

  { if (-M $f > 30) {unlink $f} }

to affect files modified more than 30 days ago.