1) Don't think about it! Go about your daily life without thinking about the number of days, hours, minutes, and seconds until launch. Putting it at the back of your mind will allow you to both A) Focus on "real" work, and B) Stop you from going crazy.
2) Play the other two! So you can't play Halo 3 yet - who cares! If you have the previous two Halo games, there's nothing better than sitting back and enjoying the past while you wait for the future. Hate Halo 2? Pop in Halo 1 and try some of this stuff after you've beaten the campaign for a little more to do.
3) Get off of the Internet! Obviously when it comes to a Halo game, the internet is not a great place to be when nearing launch. Leaks plagued Halo 2 three years ago, and now threaten Halo 3. And with the risk of Halo 3 itself getting leaked (being that it's gone gold and is now out in the wild to manufacturers' hands), staying away from possible spoilers and coverage of the game is a good idea. Even if you intentionally avoid leaks.. just gaming websites' coverage (or even Bungie.net teases) of the game leading to launch can be maddening. Especially if it's a "hands on report" - jerks.
4) Don't count the days! Counting the days until release can just make the time until launch seem that much longer. If you really have to count anything at all, try either counting just the weeks until launch. Or if you're a day person, just count the school/work days and leave out weekends and/or holidays. Those always fly by anyway.
5) Work on a website! Ok, so this one is pretty much the exact opposite of number 3, but it sure works! This doesn't have to mean doing admin work on a site like ours; this could also apply to fan content. Wallpapers, artwork, fan-fiction, machinima, etc. All take a tremendous amount of time and work - time and work that distract you from the release of Halo 3. Definitely better than going cold turkey if you're a fanatic like much of the staff on the site are, and in the end you're actually accomplishing something others can enjoy!
Doing some or all of the above tactics can help ease the painful wait until Halo 3, and make the few weeks left until launch just fly by. Or, you can ignore all of our suggestions and twiddle your thumbs until release and count every second that goes by just because that's one second closer to Halo 3. Sorry, but I'm just gonna stick to the above!
If you have a time-passer/suggestion you'd like to see added, leave it in the comments!