Whether I'm fighting the seemingly undefeatable hordes of... the Horde, or battling against hordes of easily defeatable world creatures, I find than my truest, greatest enemy is my own self. Indeed, the greatest weapon my self uses against me is that little voice which comes into my head whenever I finish a battle or a quest -- it speaks those tempting, ever-so seductive words: "Just one more..."
If I've just lost a battle in PvP, the thinking is, "Just one more -- maybe the next one will be a winner! If you log off now, you'll miss it!" But if I've just won a battle, the voice says, "Just one more -- you just won a match, so chances are you'll win the next one too!" Whenever I review my progress with honor points, gold or experience, to see how I've done this session, the voice insists: "Just a few more, and your goal of <insert however many points you think you need here> will be complete!" Even if I've firmly decided that I'm fighting no more battles of any sort today, there's always the temptation to log in to "just one more" alt to check mail, auction items, or chat with a friend for a while longer. Just logging off for good can take a long time if I'm not firm with myself, and naturally, whatever I was going to do afterwards (often sleeping) may suffer for it.
By far the best weapon I have discovered for fighting off this insidious enemy inside my own head is to decide beforehand exactly how many battleground matches I am going to play or quests I'm going to do, no matter whether I'm ultimately successful or not. This works better for me than, "I'm only going to play for <insert amount of time here>", because it is based more on exact activities I have planned. With a time limit, the little voice will say, "But I just have a little bit left to finish!" but with the activity limit, once my number is up, this voice will assuredly say "just one more" again, but then I can reply, "NO! Three is the limit! End of story! Go to sleep!" I can then click on "exit game" with glee at my complete and total victory over... me.