The good folks over at io9 have some suggestions for your 2010 Comic Con costume here. Remember, there are big prizes to be won at the Masquerade!
A few of my favorite suggestions include pretty much anything from Venture Bros (or Archer, though not mentioned here) (and h/t to Rich for introducing me to both).
Dressing up small children will make them much more tolerable as you hold up the packed aisles with strollers and screaming babies. I'm all for indoctrinating the young, but it's a tough scene for adults, let alone the newbs.
Finally, anything that is just ahead of the curve or right on time is always good. This year, it's gotta be costumes inspired by Kick Ass and Scott Pilgrim. If you're lucky, you might get some Walking Dead zombies (oh yeah, be on the lookout for the zombie parade through the Gaslamp!). However, this also means that anything that was over-exposed in the last year or 2 is O-U-T (Watchmen, Heath Ledger Joker, others?).
Take and post some of the best costumes you see, please! It's one of my favorite parts of Comic Con -- there are some really creative, talented geeks out there, and this is their time to shine.