1) Create a graphic (200 x 200 max size) to represent your personal "candy". It should have your username on it, but otherwise can feature whatever you want. Make it something special since it's self-representative.
2) Make a post with the subject "trick or treat?". Put your "candy" somewhere in it, and be sure to repost these instructions.
3) Then, go around other people's LJs and reply to them with either "trick" or "treat". If you reply with "trick", they will give you an LJ/Plurk/AIM/etc. dare that you have to perform before taking their candy. If you're too wimpy for that, simply say "treat" and take their candy.
4) List all your collected candies in your original "trick or treat?" post to show off your collection!