My technique:

1. Do not sign up first thing. Wait a few days - not so long that it's last minute and the archive's going down like The Younger Anthropologist on The Taller Cop, but long enough that a few hundred people were there first.

2. Look at the signup summary. In particular, look at the reds and yellows. Read all the red fandoms, and keep going through the yellow if you haven't already seen lots of things you liked.

3. Offer the first four red or yellow fandoms that make you actively delighted with the idea of writing a story: if someone told you that you had to write 1000 words in that fandom right now, you would be okay with that.
3a. First check what characters were nominated to see if that changes your mind. If the fandom is Stargate: Atlantis and evidently only one person nominated it, since the only characters listed are Lorne and Parrish, and you're a really big Ronon and Teyla fan, pass that one by.

4. Offer more fandoms only if you're feeling really energetic. Bear in mind that offering a zillion fandoms won't actually make the matching process run smoother (if I understand how it works correctly.)

This won't work if everyone does it (because of step 1) and it won't work if you're set on getting lots and lots of comments (because you'll be offering a less popular fandom) but it makes things a bit more manageable if your friends have already held an intervention about the spreadsheet thing and you called upon a power greater than yourself to restore you to sanity.