(Editorial Note: I'm not actually Alerting or Spoiling here. So read on!)
"Spoiler Alert! The Land of Oz was just Dorothy's dream!"
I appreciate the effort here, but it hasn't done anyone a bit of good. By the time the mind has processed the words Spoiler Alert, the Spoil has already happened. The practice, as I've seen it, must be refined.
I'm here to help.
Picture a Corvette driving down the highway at 70 mph. If you decide to jump in front of it, you better get out in front of it a little bit if you expect it to stop. Same goes with reading. Even the sharpest minds will take one or two seconds to process the words Spoiler Alert. Therefore, the Spoiler must be, at a minimum, 2 seconds after the Alert in order to do some good. Otherwise no Alert has happened. Just the Spoiling. And that's what we're trying to avoid.
You also must think of your audience when deciding the proper distance to place the Spoiler from the Alert. Let's say your audience is mainly made up of men who only read the sports page and TV Guide. These guys aren't likely to read like the Corvette speeding down the highway. Picture, instead, a guy pushing a wheelbarrow carrying 200 apples down a dirt road. Just because he travels slowly, it doesn't mean the man will be more apt to slow down for warnings. He's concentrating on not dropping the apples all over the ground. So, even for the slower, clumsier reader, it will take some time for the Alert to kick in. Thus, if you have inside knowledge of who won the Heisman, maybe think about placing the Alert a good 4 or 5 sentences before the Spoiling.
Finally, your human readers are weak-minded. We want to spoil surprises. Remember that the reader doesn't know whether it's a super awesome surprise (like Who is Keyser Soze) or a not-so-great surprise. Give us a little more warning than just Spoiler Alert. Spend a paragraph explaining why we might not want to spoil this. You have some neat information, there. If it's worth the trouble of you writing about it, maybe you shouldn't ruin it for the rest of us weak-minded fools by not being the Spoiler in the first place.
My apologies if I just ruined the surprised because you haven't seen the classic 1939 film referenced above. Spoiler Alert. I'm not talking about Gone with the Wind.