There are really only two entities you need to memorize:

jeffa++! Yes, that works if you are normal-lazy and not compulsive-obsessive like moi ;-). I hanker for orthogonality in such things and the mere idea of doing the opening tag but not the closing tag fills me with horror ;-/.

The nice thing about your comment -- besides its correctness -- is that the code (unlike a Post-It note stuck to the monitor) is extensible to tell you what you think you need to know:

 perl -e'
  print "The troublesome entities, irksome to Monks:\n  "
     , (  join " and ",
        map{
        sprintf qq{$_ is safe as }.q{&#}.qq{%d;}, ord ;
           } qw/[ </  )
     , "\n";
  '
[download]

Hopefully future readers will grasp that this is meant as somewhat of a joke: of course it's going to be easier (or more efficient, or something) for most people to simply memorize the entities. The point is that some of us have poor memories and a $10 solution to a 5-cent problem helps get around that ;-) -- If Adrian Monk was a coder, he'd be a Perl Monk, wouldn't he.